Invited Review

International Harmonization of
Nomenclature and Diagnostic Criteria(INHAND): Nonproliferative andProliferative Lesions of the Dog

Toxicologic Pathology
2021, Vol. 49(1) 5-109ª The Author(s) 2021
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DOI: 10.1177/0192623320968181
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Jochen Woicke1 , Muthafar M. Al-Haddawi2, Jean-Guy Bienvenu3,
Jessica M. Caverly Rae4 , Franck J. Chanut5 , Karyn Colman6 ,John M. Cullen7, Wendell Davis8, Ryo Fukuda9, Maike Huisinga10 ,Ursula Junker Walker11, Kiyonori Kai12, Ramesh C. Kovi13,14 ,Nicholas P. Macri15 , Heike-Antje Marxfeld10 , Kristen J. Nikula16,
Ingrid D. Pardo17 , Thomas J. Rosol18, Alok K. Sharma19 ,Bhanu P. Singh20, Kazutoshi Tamura21, Michael S. Thibodeau22,
Enrico Vezzali23, Justin D. Vidal24, and Emily K. Meseck (GESC Liaison)25

Abstract
The INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions) Project (www.toxpath.org/
inhand.asp) is a joint initiative of the societies of toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP), andNorth America (STP) to develop an internationally accepted nomenclature for proliferative and nonproliferative lesions inlaboratory animals. The purpose of this publication is to provide a standardized nomenclature for classifying lesions observed inmost tissues and organs from the dog used in nonclinical safety studies. Some of the lesions are illustrated by color photomicrographs. The standardized nomenclature presented in this document is also available electronically on the internet (http://
www.goreni.org/). Sources of material included histopathology databases from government, academia, and industrial laboratories
throughout the world. Content includes spontaneous lesions, lesions induced by exposure to test materials, and relevant infectious
and parasitic lesions. A widely accepted and utilized international harmonization of nomenclature for lesions in laboratory animals
will provide a common language among regulatory and scientific research organizations in different countries and increase andenrich international exchanges of information among toxicologists and pathologists.
Keywords
dog, pathology, toxicopathology, nomenclature, background findings, INHAND

1

Scottsdale, AZ, USA
Pathology, Research and Development, Bristol-Myers Squibb, New
Brunswick, NJ, USA
3Charles River Laboratories Montreal ULC, Senneville, Quebec, Canada
4
GlaxoSmithKline, Collegeville, PA, USA
5
Preclinical Safety, Sanofi, Chilly Mazarin, France
6
Genomics Institute for the Novartis Research Foundation, La Jolla, CA, USA
7
North Carolina State University College of Veterinary Medicine, Raleigh, NC,USA
8Alnylam Pharmaceuticals, Cambridge, MA, USA9
Axcelead Drug Discovery Partners, Inc, Fujisawa, Kanagawa, Japan
10
BASF SE, Ludwigshafen, Germany
11
Olten, SO, Switzerland
12
Daiichi Sankyo Co, Ltd, Medical Safety Research Laboratories, Edogawa-ku,
Tokyo, Japan
13
Experimental Pathology Laboratories (EPL), Inc, Research Triangle Park, NC,
USA
2

14

National Toxicology Program (NTP), US National Institute of Environmental
Health Sciences (NIEHS), Research Triangle Park, NC, USA
15
Covance Laboratories Inc, Somerset, NJ, USA
16
Inotiv, Maryland Heights, MO, USA
17
Global Pathology, Pfizer Inc, Groton, CO, USA
18
Ohio University Heritage College of Osteopathic Medicine, Athens, OH, USA
19
Covance Laboratories Inc, Madison, WI, USA
20
Gilead Sciences, Inc, Foster City, CA, USA
21
Pathology Department, BoZo Research Center Inc, Shizuoka, Gotemba, Japan
22
Boehringer Ingelheim Pharmaceuticals Inc, Ridgefield, CT, USA
23
Idorsia Pharmaceuticals, Allschwil, Switzerland
24
Charles River Laboratories, Mattawan, MI, USA
25
Novartis Pharmaceutical Corporation, East Hanover, NJ, USA
Corresponding Author:
Jochen Woicke, 10902 E Santa Fe Trail, Scottsdale, AZ 85262, USA.
Email: woicke@me.com

6

Toxicologic Pathology 49(1)Table of Contents
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.

Introduction
General (Multisystemic) Pathology
Cardiovascular System
Digestive System
Endocrine System
Hematolymphoid System
Hepatobiliary System
Integumentary System
Mammary GlandNervous System
Female Reproductive Tract
Male Reproductive System
Respiratory System
Skeletal System and Tooth
Soft Tissue and Skeletal Muscle
Special Senses
Urinary System

1. Introduction
The INHAND Project (International Harmonization of
Nomenclature and Diagnostic Criteria) is a joint initiative of thesocieties of toxicologic pathology from Europe (European Society of Toxicologic Pathology—ESTP), United Kingdom (British
Society of Toxicological Pathologists—BSTP), Japan (JapaneseSociety of Toxicologic Pathology—JSTP), and North America(Society of Toxicologic Pathology—STP) to update the existing
World Health Organization (WHO)/International Agency for
Research on Cancer and Society of Toxicologic Pathology(STP)/Standardized System of Nomenclature and Diagnostic Criteria nomenclature systems. The INHAND nomenclature and therelated diagnostic criteria represent a consensus of experienced
toxicologic pathologists and were reviewed by the INHANDGESC (INHAND-Global Editorial and Steering Committee) forcompliance with INHAND principles. Members of the societiesof toxicologic pathology had the opportunity to comment to thedraft version during a 60-day period. The initial series of nomenclature publications were focused on lesions in rats and mice.
With interest of the US Food and Drug Administration in the useof published terminology standards and the decision of the Clinical Data Interchange Standards Consortium initiative on Standard for the Exchange of Nonclinical Data (SEND) to model thecontrolled terminology (CT) based on the INHAND nomenclature, the INHAND project was extended to other laboratory animal species including the monkey, rabbit, mini-pig, fish, and dog.
Although the INHAND nomenclature and diagnostic criteria represent a preferred international standard nomenclature
for lesions identified in nonclinical studies, recommendations
for diagnostic criteria and preferred terminology may not beapplicable in all situations. The purpose of specific experiments or the specific context of a given study may require
modifications to this standardized nomenclature and diagnostic
criteria. The appropriate diagnoses are ultimately based upon
the scientific judgment of the study pathologist.

The present publication provides standardized terms anddiagnostic criteria to be used in nonclinical toxicology studies
conducted in laboratory beagle dogs. Throughout this publication, lesions applicable for use in toxicology studies in dogs aretabulated by organ system. The terms and tabulations are built
on the existing INHAND rodent nomenclature. In most
instances, the description and definition of the rodent lesion
also applies to the dog and therefore is not further described.
This publication focuses on lesions that are unique to the dog
and are not observed in rodents and lesions in dogs that sharethe same terminology with a rodent lesion but display different
morphologic features. Lesions that are unique to rats or miceand are not to be used in dogs are denoted accordingly in thetabulation. The tabulated lesions are categorized according to
the following characteristics: “common,” “uncommon,” “notobserved but potentially relevant,” and “not applicable.”
The distinction between common and uncommon lesions isbased on the occurrence in untreated beagle dogs in theauthors’ experience and is not based on published references.
The uncommon category is reserved for changes that are
observed only sporadically as spontaneous findings in mostdog studies or those that are induced almost exclusively byxenobiotics. “Not observed but potentially relevant” arechanges that have not been described or observed in dogs;
however, the use of these terms has been considered permissible, should a lesion meet the diagnostic criteria. The category“not applicable” refers to lesions and terms specific to rodents,
such as chronic progressive nephropathy in the kidney; the useof these terms in dogs is not considered appropriate. It should
be kept in mind that the dogs used in toxicology studies areusually of young age and are only on study for a relatively shorttime (usually not more than 52 weeks), a fraction of the normallife span of dogs. The health status of individual dogs is usuallychecked carefully, and the individual dogs selected for a study
are in excellent condition. For these reasons, spectrum andfrequency of changes observed in this population are different
from those in diagnostic laboratories, and common age-related
lesions are rarely seen. Because neoplasms represent a veryrare event in toxicity studies in dogs, neoplasms are generally
excluded and only included if they are of relevance, that is, if
the finding has been recorded in a toxicity study. For a description and the diagnostic criteria for tumors in dogs, the reader isreferred to the fascicles of the International Histological Classification of Tumors of Domestic Animals published by the
Armed Forces Institute of Pathology in conjunction with theAmerican Registry of Pathology and the World Health Organization, the textbook Tumors of Domestic Animals,1 and/or
current literature. Whenever possible, the equivalent rodent
term/SEND terminology should be used for any tumors not
specifically addressed in this manuscript, as appropriate.
In addition to the journal publication, the nomenclature anddiagnostic criteria for the dog are also available online.2 The
online version contains additional images and useful links todifferential diagnoses characterizing it as a practical tool for
diagnostic work. In addition, all INHAND publications are available at the website of the Society of Toxicologic Pathology.3

Woicke et al.
Several manuscripts and texts have been published on background lesions in the laboratory beagle, and the reader is directedto the general bibliography at the end for further reading.
The recommended nomenclature is generally descriptive
rather than diagnostic based on standard hematoxylin andeosin–stained paraffin-embedded sections only. Histochemical
or immunohistochemical staining characteristics may be
addressed in the comments section of the respective lesion.
Such special techniques may be required in some situations,but a comprehensive discussion of these methods is outside thescope of this publication. Systemic nonproliferative lesions that
occur across organs systems and are not specific to an organ are
reviewed in the section on systemic pathology. Instead of“synonyms” for each term, as was used in some earlier rodent
publications, the nonrodent publications have used the notation“other term(s).” While these synonyms or other terms havebeen used historically, the primary listed term is the preferredterm and will link to the CT in SEND.
Findings included in this nomenclature system may be further specified by modifiers. Criteria are given for modifiers thatare of particular relevance. These modifiers should be consistently applied. Additional modifiers not provided in thisnomenclature system may describe the location, tissue type,or duration, among others. General principles of the INHAND
nomenclature have been published separately.4 As an excellent
resource for the pathology in dogs, see the textbook on Pathology of Domestic Animals,5 which also provides extensive reference lists. As new information becomes available, new terms
will be needed from time to time and a request for this new term
will be applied by “change control.”2,3

7

Diagnostic Features
Predominantly normal tissue in an abnormal location.

Comment
In most instances, ectopic tissues are asymptomatic and represent an incidental finding during necropsy or microscopic
examination.

Edema—Multiple tissues
Other Term(s)Not applicable.

Pathogenesis/Cell of Origin
Increased permeability of the vascular system leading to excessive accumulation of vascular fluid in tissues due to







increased hydrostatic pressure in venous capillaries
while an unchanged colloid–osmotic pressure is maintained (stasis)
decreased colloid–osmotic pressure while blood pressure remains normal (hypoalbuminemia)
damage to the capillary endothelium (inflammatory,
toxic, allergic, hormonal dysregulation)
obstruction of lymph drainage
a combination of the above (a coincidence of more than
one underlying reasons is frequent)

2. General (Multisystemic) Pathology
Findings that may be observed in multiple tissues are listed inTable 2.1 with associated diseases and conditions as well as
tissues, in which they are usually observed. An associated comment or description is available in this chapter for terminology
marked with an asterisk. For description of other terminology,
the reader is referred to the chapter of the corresponding organs.

Ectopic Tissue—Multiple Tissues

Diagnostic Features
Swelling of the affected tissue; microscopically, the structuresappear less dense.

Differential DiagnosesOther Term(s)Accessory (tissue).

Pathogenesis/Cell of Origin
Pathogenesis varies, depending on the originating tissue organ,
for example:

Shedding off of fragmented adrenal tissue during
development
Failure of appropriate migration or involution during
development (thyroid gland, thymus)
Acquired autoimplantation after trauma or surgery(spleen)Inflammation: Presence of inflammatory infiltrates.
Fibrosis: Increase in collagenous or elastic fibers.
Lipoproteinous (alveolar): Lipoproteinous is, unlike
edema, characterized by eosinophilic, Periodic acid–
Schiff (PAS)-positive exudate.

Comment
Edema is frequently noted in the subcutaneous tissue and thelungs but can occur in any organ and tissue.

Hemorrhage—Multiple Tissues
Other Term(s)

Bleeding
Extravasation

8

Toxicologic Pathology 49(1)Table 2.1. Microscopic Findings of Systemic Pathology (Generally Used Preferred Terms): Dog.
LesionCommon Uncommon Associated diseases/conditions

Multisystemic congenital
Aplasia
Ectopic tissuea

xMultisystemic nonneoplastic
Accumulation, adipocytesxx

Amyloid
Apoptosisb
Congestionxx
x

EdemaaExtramedullary hematopoiesis (EMH)cFibrosisHemorrhageaxxx

Mineralizationx

Repair process
Increased permeability of blood vessels
Traumaxx

Response to injury
Chronic inflammation
Ischemia
Hematoma
Degenerative changes
Local injury or generalized mineral
imbalance

Necrosis
Necrosis/inflammation, medial or muralxx

ParasiteaxSerous atrophy of fatd

Inflammation
Hypertension
Cardiac decompensation
Increased vascular permeability
Anemiaxx

Infiltrate [insert appropriate cell type]aInflammationa
Metaplasia, osseous, or cartilaginous

Pigment

Developmental anomaly
Developmental anomalyxx

Tissues commonly reported in:
Kidney (unilateral)
Adrenal cortex
Pancreas
SpleenThyroid
Thymus
Pancreas
Skeletal muscle
ThymusThyroid
Adrenal gland
KidneyLiverMultiple tissues
Liver
Lung
SpleenMultiple tissues
SpleenLiver
Multiple tissues
Multiple tissues
Multiple tissues
Multiple tissues
Dura mater
Lung
Parathyroid gland

KidneyOvary
Brain
Leptomeninges
Blood vessels
Lung
Salivary glands
StomachMultiple tissues
Idiopathic canine polyarteritis, beagle pain Blood vessels
syndrome
Coronary arteries
Mediastinal arteries
Meningeal arteries
Cervical spinal arteries
Intestines
Skin
SpleenAccumulation of endogenous or
Liverexogenous pigment, eg, hemosiderin,
Bone marrow
hematoidin, lipofuscin, ink (tattoo)
Adrenal gland
KidneyLymph nodes
Skin
Brain
Chronic emaciation and/or inanition
Bone marrow cavity
Heart
Perirenal tissue
Soft tissue(continued)

Woicke et al.

9

Table 2.1. (continued)
LesionCommon Uncommon Associated diseases/conditions

Thrombus

Vacuolationa
Systemic neoplastic
LymphomaexxIntravenous injection or infusion
Inflammation
Necrosis
Infarct
SepsisCoagulopathies
Associated with various conditions andadministration of xenobiotics

Tissues commonly reported in:Blood vessels: veins

Multiple tissuesxaTerminology with diagnostic criteria and/or comments described in the text.
For the use of apoptosis and single-cell necrosis, refer to the publication recommendations from the INHAND Apoptosis/Necrosis Working Group (seedescription/citation in the hepatobiliary system).
cRefer to the description under general hematolymphoid.
d
Refer to the description of atrophy under soft tissue in the rodent manuscript.
e
For diagnostic criteria of neoplastic lesions, refer to the “International Histological Classification of Tumors of Domestic Animals,” the more recently published“Tumors of Domestic Animals”1, and other pertinent literature.
b

Pathogenesis/Cell of Origin
Extravasation of red blood cells.

Infiltrate [insert appropriate cell type]—
Multiple Tissues
Other Term(s)Diagnostic Features

Aggregates, inflammatory cell.

Red blood cells in the parenchyma, interstitial space, or body
cavity, outside the vascular system.

Modifier

Differential Diagnoses
Congestion. Accumulation of blood within the vascular system.

Eosinophil, (histiocyte), lymphocyte, macrophage (preferredterminology over “histiocyte”), mixed cell, mononuclear cell,
neutrophil, plasma cell, polymorphonuclear cell.

Diagnostic Features
Comment
Hemorrhage, bleeding, extravasation, or the escape of bloodfrom the blood vessels may be observed in virtually everyorgan and tissue and is microscopically characterized by the
extravasal presence of erythrocytes in tissues. Usually, the following etiologies are discerned:






TraumaErosionRuptureDiapedesis
Asphyxia

For detailed descriptions of the various types, the reader
is referred to textbooks of general pathology. Most relevantin toxicologic pathology is hemorrhage by diapedesis,
caused by, for example, allergic reactions, infections, toxic
agents, thrombocytopenia, disorders of blood coagulation,and so on. Extravasation of red blood cells in areas of
exsanguination as part of the euthanasia process is generally
not recorded as part of a general toxicology study as this isconsidered an agonal and procedural finding, unrelated tothe objective of the study.

Foci of cell infiltrates
Absence of tissue damage

Differential Diagnosis
Inflammation: In addition to the inflammatory cell infiltrates,
additional features are edema, tissue damage, hemorrhage, and/or fibrosis.

Comment
Frequently used term to describe the presence of inflammatory
cells without significant accompanying inflammation, tissue
damage, or morphologic evidence of cellular injury, noted ina variety of organs and tissues. May be within limits of normal.
The term should be used with a descriptive modifier of the cell
type, for example, infiltrate, lymphohistiocytic.

Inflammation—Multiple Tissues
Other Terms“-itis,” specific to the organ affected, for example, encephalitis,
gastritis, pneumonia, and so on.

10

Toxicologic Pathology 49(1)Comment
For organ-specific, detailed characteristics, refer to the description in the respective organ section.
Various approaches to further characterize inflammation areused by pathologists, including chronicity (peracute, acute,subacute, etc), location (perivascular, peribiliary, etc), and others. To implement a descriptive terminology, the use of theterm “inflammation” is preferred over the “-itis” terminology,
and the indication of the predominant cell type(s) in the diagnosis is recommended over the conventional use of chronicity,
for example:Inflammation, neutrophilic
Inflammation, lymphocytic
Inflammation, plasmocytic
Inflammation, histiocytic

When applicable the terms are combined, for example:
Inflammation, lymphoplasmocytic
Inflammation, lymphohistiocytic

Further description by location and distribution isrecommended.

Parasite—Multiple Tissues
Comment
For details on the diagnostic criteria of parasites, the reader isreferred to textbooks on veterinary parasitology. While parasitic
structures can be observed in multiple organs and tissues ofdomestic dogs, their occurrence in laboratory dogs used in toxicity studies is very limited due to the stringent hygiene conditions in breeding facilities and laboratories. Parasites that may be
noted occasionally include the nematode Toxocara canis and thearthropods Demodex canis and Sarcoptes scabiei. When parasites are observed, they should be recorded as “parasite” and, if
possible, specified further in a comment to the finding. Parasite
may be recorded as “present” or graded depending on the datacollection system used in the toxicology study.

Vacuolation—Multiple Tissues
Other Term(s)Fatty change, lipidosis, lipid accumulation, phospholipidosis,
and so on.

Comment
The descriptive term vacuolation is the preferred term over the
etiologic terminology listed above. It is considered good practiceto add modifiers such as lipid, fatty change, or phospholipidosis
if the nature of the vacuoles has been confirmed by special
methodology, for example, lipid stain or electron microscopy.
In the case of lipid vacuolation, micro- and macrovesicularvacuolation may be also indicated by modifier or comment.

3. Cardiovascular System—Heart, Heart
Valves, and Blood Vessels
Introduction
Review of the incidence of spontaneous cardiovascular lesionin dogs has been published,6,7 and protocol for the collection
and dissection of the heart in dogs is available.8 For detailed
general considerations of the cardiovascular system, refer tothe INHAND publication on the rodent cardiovascularsystem.9
In nonclinical studies, both morphological and functionalend points are essential in the identification of any potential
interaction between drugs and cardiovascular structures. As aminimum, the standard morphological evaluation of the cardiovascular system includes heart weight measurement andconventional light microscopic histopathology of the myocardium and valvular leaflets in the heart, aorta, and bloodvessels within tissues. Evaluation of serum levels of heartspecific biomarkers, electron microscopy, and immunohistochemistry may represent essential investigative tools to better
identify the pathological process caused by xenobiotics.
Functional assessments, such as blood pressure and echocardiography, can provide valuable in-life information correlatingwith morphological alterations observed at the postmortem
examination.
Additional structures of the conduction system may also be
evaluated following careful sampling.10 The microscopic evaluation of the vascular structure in the other organs is performed as partof evaluation of each specific tissue. This may reveal the presenceof either organ-specific vascular changes (possibly part of anorgan-specific toxicity) or widespread vascular changes visible inseveral organs pointing toward a systemic vascular injury.

Heart
Introduction. At necropsy, similar to other lab animal species,
the heart is sampled with the root of the large vessels and fixedin buffered formalin solution. The heart is generally opened
prior to immersion in the fixative in order to ensure adequate
fixation and eliminate large blood clots occupying the cardiac
chambers. Conventional light microscopic histopathology ofthe heart is based on a thorough and consistent microscopic
evaluation of hematoxylin and eosin–stained section of all relevant compartments and structures of the heart, including theventricular, atrial, and septum wall, the valves, and the coronary vessels. This allows an accurate identification of changes in
the muscular cells, extracellular matrix, conduction system
within the myocardium, and the vascular structures within the
myocardium and the adjacent epicardial tissue. Attention
should also be given to regional distribution of lesions within
the heart. For example, subendocardial zones in the left ventricle and left papillary muscles, especially near insertion siteof chordae tendineae, are especially vulnerable to ischemicbased myocardial lesions.11,12
The recommended terminology of microscopic lesions
observed in the heart of dogs is presented in Table 3.1.

Woicke et al.

11

Table 3.1. Microscopic Findings of the Cardiovascular System: Heart; Dog.
Heart
Nonneoplastic
Amyloid
Apoptosis, cardiomyocytea,bCardiomegaly
Cardiomyopathy, rodent progressive
Degeneration, cardiomyocyte
Degeneration/necrosis, cardiomyocyte
Edema, myocardium
Fibrosis, myocardium
Hemorrhageb,cHypertrophy, cardiomyocyte
Infarct
Infiltrate [insert appropriate cell type], myocardium
Infiltrate/fibrosis
Mineralization, cardiomyocyte, or myocardium
Mononuclear cell infiltrate/fibrosis, myocardium
Necrosis, cardiomyocyte
Necrosis/infiltrate, cardiomyocyte
Pigment, cardiomyocyte, or myocardium
Rodent progressive cardiomyopathy
Thrombus, atrium
Vacuolation, cardiomyocyte
Vacuolation, conduction systemb,cProliferative (nonneoplastic)
Hyperplasia, mesothelial, epicardium, or pericardium
Hyperplasia, Schwann cell, subendocardium
Neoplasticd
Rhabdomyoma, myocardiumc

CommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

Finding more frequent as an induced change.
Terminology addressed in the General Pathology Section.
Terminology with diagnostic criteria and/or comments described in the text.
d
For a comprehensive list of neoplasms and tumor-like lesions, refer to the “International Histological Classification of Tumors of Domestic Animals,” the more
recently published “Tumors of Domestic Animals”1, and other pertinent literature.
bcHemorrhage—Heart, Heart Valve

Diagnostic Features

Comment

Round (spherical) empty spaces (vacuoles) usually centrally
located in Purkinje fibers.

General aspects and a description of hemorrhages are includedin the General Pathology section. In the heart, subendocardial,
epicardial, and valvular hemorrhages (“heart valve
hematomas”) are often observed and readily noted at necropsy.
In addition, myocardial hemorrhages may be noted microscopically. It is recommended to indicate the location as descriptor
or modifier to the term.

Differential Diagnoses
Not applicable.

Comment
Vacuolation, Conduction System—Heart
Other Term(s)Not applicable.

Pathogenesis/Cell of Origin
Purkinje fiber.

Vacuolation of the conduction system is occasionally observedin beagle dogs. In a publication on morphologic evaluation of
the heart in dogs and monkeys, Keenan and Vidal7 reported
prominent vacuolation of Purkinje fibers in 15% of the males
and 12% of the females, while Bodié and Decker6 reported thischange in 34% of the male and 28% of the female control dogs.
Authors also noted that vacuolation varied in prominence but
was more evident in fibers following vascular adventitia into
the ventricles.

12

Toxicologic Pathology 49(1)Rhabdomyoma, Cardiac—Heart
Other Term(s)Rhabdomyomatosis; congenital glycogenic tumor.

Pathogenesis/Cell of Origin
Exact histogenesis is uncertain, although conducting fiber,
myocardial fiber, and pluripotent embryonic cells are considered to be origin.

Diagnostic Features







Tumor is well circumscribed, expansile, andnonencapsulated.
Solitary or multiply occurrence in myocardium.
Tumor consisted of tightly arranged, ovoid to irregularswollen cells that have distinct cell borders with a deeply
eosinophilic cytoplasm and varying degrees of cytoplasmic vacuolation and single, oval to elongate, peripherally located nucleus with 1 or 2 prominent nucleoli.
Tumor cells contain abundant cytoplasmic glycogen andthe so-called spider cells occasionally appear.
No mitotic activity.

Differential Diagnoses

Rhabdomyosarcoma: Highly pleomorphic and highmitotic activity with abnormal mitotic figures. Locallyinfiltrative with frequent distant metastases. Frequently
showing necrosis and hemorrhage.
Lipoma: Well-demarcated, lobulated mass. Consist of
mature fat cells containing a single fat vacuole witheccentrically located nucleus. Tumors are usually separated into lobules by fibrous septa.
Glycogen storage disease (so-called glycogenosis): No
well-circumscribed nodules. This disease is characterized by glycogen accumulation in the heart, skeletal
muscles, liver, kidneys, or muscular layer of esophagus.

Special Techniques for Diagnostics
The PAS reaction with or without diastase digestion is applied forthe detection of glycogen in neoplastic cells. Immunohistochemically, neoplastic cells are demonstrated positively for desmin andmyoglobin but negatively for smooth muscle actin or vimentin.

Comment
Cardiac rhabdomyoma has been reported in various animals
including humans and guinea pigs. It is a controversial issue
whether cardiac rhabdomyoma is a true neoplasm or hamartoma. Human cardiac rhabdomyomas are caused by mutation in
the TSC1 and TSC2 gene. A mouse model of cardiac rhabdomyoma is associated with loss of the TSC-1 gene in ventricular
myocytes.13 One of the most interesting aspects of these tumors
is their tendency to undergo spontaneous regression. 14

Rhabdomyoma can occur in the myocardium, skeletal musclesof the larynx, and in the head region in both human and animals.

Heart Valves
Observation of drug-induced valvulopathy in postmarketingsurveillance in man has led to withdrawal of some commercialized compounds, such as appetite-suppressant drugs. This has
increased the interest in the evaluation of heart valves during
preclinical studies.
Histopathological examination of the cardiac valves in alllaboratory animal species, including dogs, may suffer frominconsistencies and artifacts related to both the technicalapproach and the inherent small size of the valves. The trimming/sectioning of the heart may greatly influence the type,
number, and correct orientation of valves within the histologicalsections, which finally affects the accuracy of the microscopic
examination. Therefore, in those studies where drug-related
effects are suspected to occur in the cardiac valves, the pathologist may need to adapt the preparation of the heart and trimming of valves to ensure the best representation of most or all
4 cardiac valves. Although congenital valvular anomalies (pulmonary stenosis, aortic stenosis) can be observed in dogs,15
these are very uncommon in beagle dogs purpose-bread for biomedical research and used in toxicity studies.
The recommended terminology of microscopic lesions
observed in the heart valves of dogs is presented in Table 3.2.

Degeneration, Myxomatous, Valve—Heart—Heart Valve
Comment
Myxomatous valvular degeneration is a common valvularlesion in older dogs, affecting chiefly the left atrioventricularvalve.15 Macroscopically, the valve may appear irregular,
shortened, thickened, opaque, with discrete to prominent
nodules that may extend to chordae tendineae. Microscopically, thickening of the spongiosa by loose fibroblastic
glycosaminoglycan-rich tissue and degeneration of the
fibrosa with often hyalinized and fragmented collagen fibers
are prominent features. The presence of myxomatous tissue atthe base of the heart, near the aortic valve and aorta, is acommon normal component of the heart of dogs7 and should
not be confused with myxomatous degeneration.

Hemorrhage—Heart—Heart Valve
Comment
General aspects and a description of hemorrhages are includedin the General Pathology section. In the heart, subendocardial,
epicardial, and valvular hemorrhages (“heart valve
hematomas”) are often observed and readily noted at necropsy.
In addition, myocardial hemorrhages may be noted microscopically. It is recommended to indicate the location as descriptor
or modifier to the term.

Woicke et al.

13

Table 3.2. Microscopic Findings of the Cardiovascular System: Heart Valve; Dog.
Heart valve

CommonNonneoplastic
Angiectasis, valvular
Degeneration, myxomatous, valve*
Hemorrhagea,bInflammation, valveb
Proliferation, stroma, valve
Neoplastic
Myxoma, valvea
Myxosarcoma, valveaab

UncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x

Terminology with diagnostic criteria and/or comments described in the text.
Terminology addressed in the General Pathology section.

Myxoma, Valve—Heart Valve

Myxosarcoma, Valve—Heart Valve

SpeciesSpeciesDog (term does not exist in rodent goRENI).

Dog (term does not exist in rodent goRENI).

Other Term(s)Not applicable.

Other Term(s)Not applicable.

Pathogenesis/Cell of Origin
Multipotent vasoformative cells originating in subendocardial
layer.

Pathogenesis/Cell of Origin

Diagnostic Features

Multipotent vasoformative cells originating in subendocardial
layer.





Grossly, multilobular, soft, gelatinous mass; may havehemorrhagic areas; can obstruct the blood flow.
Myxomas are covered by endothelial cells and are composedof an abundant hypocellular myxoid matrix with stellate or
globular cells, blood vessels, and smooth muscle cells.
May have capillary-like channels lined by endothelial cells.

Diagnostic Features

Differential Diagnoses
Myxosarcoma: high cellular pleomorphism, presence ofnumerous mitotic figures, metastasis

Special Techniques for Diagnostics
Grossly, multilobular, soft, gelatinous mass may havehemorrhagic areas; can obstruct the blood flow.
As for the benign form, myxosarcomas are covered byendothelial cells and are composed of an abundant hypocellular myxoid matrix with stellate or globular cells,
blood vessels, and smooth muscle cells, with high cellular pleomorphism, numerous mitotic figures, and areasof necrosis. Areas of cartilaginous or osteoid-like differentiation can also be present.
Presence of metastasis in lymph nodes and viscera.

Histochemical stain such as PAS and Alcian blue stain may be
used to confirm the acid mucopolysaccharide nature of the
myxoid matrix, and IHC may be used to confirm the presenceof endothelial cells and mesenchymal nature (vimentin-positive) of myxoma cells.

Differential Diagnoses

Comments

Special Techniques for Diagnostics

Rare in dogs (common primary cardiac tumor inhumans).
Will often disseminate by embolization.
Associated with hemodynamic consequences according
to size and anatomical location.

Myxoma—No cellular pleomorphism, mitotic figures ormetastasis.

Histochemical stain such as PAS and Alcian blue stain may be
used to confirm the acid mucopolysaccharide nature of the
myxoid matrix, and IHC may be used to confirm the presenceof endothelial cells and mesenchymal nature (vimentin-positive) of myxosarcoma cells.

14

Toxicologic Pathology 49(1)Table 3.3. Microscopic Findings of the Cardiovascular System: Blood Vessels; Dog.
Blood vessels
Nonneoplastic
Amyloid, media or wall, artery
Aneurysm, artery or aortic
Angiectasis
Degeneration/necrosis, media, or wall
Embolus
Fibrosis, perivascular
Hemorrhage, media or walla
Hypertrophy, endothelium
Hypertrophy, media or wall, artery
Infiltrate [insert appropriate cell type], perivasculara
Inflammation, media or wall, arterya
Intimal thickening, acellular, artery
Intramural plaque, artery
Intramural plaque, fattyb
Mineralization, media or wall, arterya
Necrosis/inflammation, media or wall, arterya,bThrombus
Vacuolation, media or adventitia, arterya
Proliferative (nonneoplastic)
Hyperplasia, angiomatous
Hyperplasia, hemangioendothelium
Hyperplasia, intravascular papillary endothelialb
Proliferation, intima, artery or vein
Neoplastic
Hemangioma
HemangiosarcomaabCommonUncommonNot Observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

Terminology addressed in the General Pathology section.
Terminology with diagnostic criteria and/or comments described in the text.

Comments
This tumor is extremely rare in dogs. As for the benign form,
cardiac myxosarcoma may be associated with hemodynamic
consequences according to the size of the mass and anatomical
location.

Blood Vessels
The microscopic evaluation of the vasculature in organs isgenerally performed as part of the evaluation of each specificorgan. This may reveal the presence of either organ-specificvascular changes (possibly as part of an organ-specific toxicity)or widespread vascular changes visible in several organs pointing toward a systemic vascular injury. Specific collection oflarger vessels may be required in particular when intravascularadministration of xenobiotics is performed (eg, through bolus
injection or slow infusion), with potential for local and/or systemic vascular injury. Vascular injury in dogs, as well as inother laboratory animal species, can occur as a spontaneous
pathological change or be drug induced. In preclinical drug
development, vascular injury can be caused by a wide arrayof compounds encompassing small and large molecules (suchas monoclonal antibodies) and antisense oligonucleotides. Several papers have been published to better characterize druginduced vascular injury and associated biomarkers in

preclinical studies.16–19 However, the distinction between the
spontaneous and drug-induced vascular injury may still bechallenging and a correct identification of the histological pattern of vascular changes is crucial.
The observation of a vascular injury may reflect an organspecific toxicity or represent a widespread change visible inseveral organs and, therefore, suggest the occurrence of a systemic vascular disorder.
Histopathological evaluation of the heart and the otherorgans according to standard guidelines for organ trimming is
considered sufficient to ensure the thorough evaluation of thevascular tree.
The recommended terminology of microscopic lesions
observed in the blood vessels of dogs is presented in Table 3.3.

Intramural Plaque, Fatty—Blood Vessel
Other Term(s)Atheroma, fibrofatty plaque.

Pathogenesis/Cell of Origin
Intimal/medial accumulation of lipids intermixed with inflammatory cells and extracellular matrix. Lesion typically associated with atherosclerosis.

Woicke et al.

Diagnostic Features







Grossly, affected vessels are enlarged, with thickened,
cord-like walls; plaque may protrude into the lumen.
Microscopically, consists of an accumulation of freelipids (may have cholesterol clefts) or intracellular lipids(usually within foamy macrophages) expanding themedia and extending into the intima, with disruptionof the internal elastic lamina.
Associated with variable amount of inflammatory cells,smooth muscle cells, and extracellular matrix.
Plaque may be mineralized and have a necrotic core.
Infrequently, plaque may be ulcerated and associatedwith vascular thrombosis and hemorrhage.

Differential Diagnoses
Arteriosclerosis—Thickening of the intima/media by hyalinematerial (mucopolysaccharides, fibrin) or by laminar proliferation of smooth muscle cells and extracellular matrix, without
accumulation of lipids.

Special Techniques for Diagnostics
Intramural fatty plaque may be highlighted with histochemical
stains for elastic fibers, fat, and mineralized material andimmunohistochemical stain for macrophages and smooth muscle cells.

15

Pathogenesis/Cell of Origin
Necrotizing polyarteritis; exact etiopathogenesis unknown, aTh2-mediated immune response, and upregulation of CD11a
integrin and activation of metalloproteases are suspected.

Diagnostic Features
1. Acute to chronic necrotizing fibrinoid arteritis affecting
the media to the totality of the vascular wall of small to
medium size muscular arteries of young beagles andmedium to large breed dogs; typically affects the coronary, mediastinal, meningeal, and cervical spinal
arteries but can also be observed in arteries of otherorgans.
2. Morphological appearance is variable depending on thenumber of febrile episodes and stage of the disease.
Characterized by intimal proliferation, homogenous
eosinophilic material and karyorrhectic debris in media,
transmural to periarterial infiltrates of predominantly
neutrophils with scattered lymphocytes, plasma cells
and macrophages, variable degree of hemorrhage, rupture of internal elastic laminae, medial hypertrophy, andfibrosis.
3. Followed potentially by thrombosis and infarction
within affected tissues.

Differential Diagnoses
1.

Comment
Intramural fatty plaques in arteries/arterioles, characteristically associated with atherosclerosis, are rare spontaneousfinding in dogs as dogs are resistant to the development of
atherosclerosis. However, this lesion is seen infrequently indogs following developing hypercholesterolemia associatedwith endocrinopathies.
The intramural fatty plaque in dogs will typically affect thevasculature of the heart, brain, and kidney and less often othervasculatures; deposition of lipids typically begins in the middle
and outer layer of the media, eventually extends to the intima
and affects more extensively the small muscular arteries. In
contrast, intramural fatty plaques in humans are present primarily in the intima and affect primarily the major elastic arteries(aorta, carotid) and the large and medium caliber musculararteries (coronary, popliteal).

Necrosis/Inflammation, Media or Wall,
Artery—Blood Vessel (Figures 3.1 and 3.2)
Other Term(s)Beagle pain syndrome, idiopathic canine polyarteritis, canine
juvenile polyarteritis syndrome, steroid-responsive meningitisarteritis.

Hemorrhage, vascular: Presence of extravasated erythrocytes in vascular wall without wall injury.
2. Inflammation, vascular: Infiltrates of inflammatory
cells in vascular wall without evidence of cell injury.

Comment
It is suggested to use necrosis/inflammation, medial or mural,artery to describe necrotizing inflammation in arteries. It can bedifficult to differentiate idiopathic canine polyarteritis fromdrug-induced vascular injury, especially when an increased
incidence of chronic lesion consistent with this disease ispresent following treatment with a vasoactive drug.
Vasodilator-induced lesions are generally characterized bymedial/adventitial hemorrhage and necrosis with minimal
inflammation (acute) to intimal and adventitial proliferativechanges (chronic); are typically limited to the coronary arteries(extramural and intramural), often associated with hemodynamic and myocardial changes; and may be associated withatrial hemorrhage. Vasoconstrictor-induced lesions usuallyaffect small size arteries in a variety of tissues and include
medial thickening and necrosis, with hyalinization. Hypersensitivity vasculitis (type IV) affects small vessels (not limited toarteries) of skin or of other tissues and is characterized bytransmural nonnecrotizing vasculitis with infiltrates of mononuclear and variable number of eosinophils, generally with nofibrinoid necrosis or thrombosis.
The site predilection, dose–response, and clinical andlaboratory changes can be differentiating. In drug-induced

16

Toxicologic Pathology 49(1)vascular injury, vascular lesions are generally restricted to coronary arteries, and clinical signs are variable, likely unrelated
to vascular effects. Idiopathic canine polyarteritis will usuallyshow prominent transmural to periarterial inflammation withlittle or no hemorrhage, will typically affect arteries of the heartand of many other tissues as well, myocardial changes will only
rarely be present, and idiopathic canine polyarteritis is not
associated with hemodynamic changes or atrial hemorrhage.
Clinically, acute idiopathic canine polyarteritis is typicallyassociated with pyrexia, anorexia, reluctance to move, cervicalrigidity and pain, with elevated polymorphonuclear cell countand concentration of IgA and acute phase proteins in serum andcerebrospinal fluid. In the chronic form of the disease, progressive atrophy of temporal and cervical muscles and neurologicaldeficits may occur.

Hyperplasia, Intravascular Papillary
Endothelial—Blood Vessel (Figures 3.3
and 3.4)
Other Term(s)Reactive vascular proliferation; proliferation, intravascularendothelial; angiomatosis, intravascular.

Pathogenesis/Cell of Origin
Endothelial proliferation in response to traumatic injury followed by thrombosis, inflammation, and stasis within vascularbed.

Diagnostic Features
1.

Papillary formations are totally confined to cystic
dilated vascular lumina.
2. Lesion is covered by a single layer of endothelium.
3. Fibrous pseudocapsule containing residual smoothmuscle of the preexisting vessel wall may be present.
4. Myriad of small delicate papillae projections into thelumen may be seen.
5. Inflammatory infiltrates and thrombus may be present.
6. Proliferating endothelial cells appear prominent or plump
but lack significant pleomorphism and mitotic figures.

Differential DiagnosesHemangiosarcoma: Proliferation of atypical endothelial
cells form vascular channels (capillary to cavernous)
and solid cellular masses. Mitotic figures can be common and bizarre. Local invasion and metastases areoften present.
Hemangioma: Proliferating endothelial cells show slightcytological abnormalities and form variably sized vascular spaces (capillary to cavernous) and compression ofsurrounding tissues.
Endothelial hyperplasia: Proliferation of normally present endothelial cells without papillary proliferation.

Special Techniques for Diagnostics
Immunostaining for vimentin, von Willebrand factor, andCD31 are applied for identification of endothelial cells.

Comment
Canine intravascular papillary endothelial hyperplasia has beensurveyed by Gamlem and Nordstoga.20,21 This lesion is thought
to represent an abnormal morphologic type of organizing
thrombus. Therefore, it should not be confused with preneoplastic or neoplastic lesions.22 Scrotal vascular hamartoma indogs may be the same entity of this lesion.

4. Digestive System—Oral Cavity, Salivary
Glands, Esophagus, Stomach, Intestines,
and Exocrine Pancreas
Introduction
For detailed general considerations on the digestive system,
refer to the INHAND rodent publication.23
This chapter provides a set of standardized terms, diagnostic criteria, and examples for the upper and lower digestive tract, as well as for the salivary glands and the exocrine
pancreas of beagle dogs used in toxicologic pathology
studies.
The digestive tract is the entry site into the body fororally administered test articles. An irritant test article maylead to local acute lesions at this first site of contact to thebody, and the digestive tract can be affected by adverse
drug reactions. On the other hand, orally administered test
articles usually reach systemic exposure, sufficient to be
toxic to other organs, without noticeable effect on the digestive tract.
A distinctive feature of the digestive tract is the high proliferative rate of the epithelium, making it particularly sensitive
to agents interfering with cell division but resulting also in ahigh regenerative capacity. Because of the large surface area ofthese tissues, accurate assessment of potential treatment effects
is almost entirely dependent on a thorough gross examinationand sampling of focal lesions.
For the classification of neoplastic lesions, refer to the“International Histological Classification of Tumors ofDomestic Animals: Tumors of the Alimentary System ofDomestic Animals,” published by the Armed Forces Instituteof Pathology in conjunction with the American Registry of
Pathology and the World Health Organization,24 the more
recently published “Tumors of Domestic Animals,”25 andother pertinent literature.

Upper Alimentary Tract (Oral Cavity, Tongue,
Pharynx, and Esophagus)
Most commonly, microscopic lesions in the upper alimentary tract are observed in tissues that are required to be
examined microscopically, based on regulatory guidelines

Woicke et al.

17

Table 4.1. Microscopic Findings in the Digestive System: Oral Cavity and Esophagus; Dog.
Oral cavity and esophagus
Congenital
Cleft, palatea
Dilatation, esophagus
Diverticulum, esophagus
Ectopic tissue, sebaceous glandb
Malformation
Nonneoplastic
Amyloid
Apoptosis, squamous epitheliumb,cApoptosis/single-cell necrosisb,cAtrophy, squamous epithelium
Cyst
Degeneration/necrosis, muscle
Edemab
Erosion/ulcerc
Hemorrhageb
Hyperkeratosis
Infiltrate [insert appropriate cell type]b
Inflammation, foreign body
Inflammation, mixed cell
Inflammation, mononuclear cell
Inflammation, vessel
Mineralizationb,dNecrosis, squamous epitheliumb
Pigmentb
Proliferative (nonneoplastic)
Eosinophilic granulomad
Hyperplasia, basal cell
Hyperplasia, squamous cellc
Papilloma, squamous celld

CommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

This and other malformations are rarely observed in toxicity studies and generally represent gross observations; a microscopic correlate may be required,however.
b
Terminology addressed in the General/Systemic Pathology section.
cFinding more frequent as an induced change.
dTerminology with diagnostic criteria and/or comments described in the text.

and the preferences of the laboratories, for example, esophagus, pharynx, tongue. In most instances, other lesions in
the oral cavity and pharynx are examined only when grosslesion were recorded. Table 4.1, based on the rodent manuscript, lists the nonproliferative and nonneoplastic proliferative changes of the upper alimentary tract.

Diagnostic Features

Sharply defined, irregular areas of amorphous, calcified(Kossa positive) material.
Surrounded by epithelioid and multinucleatedmacrophages.
Peripheral fibrosis.

Differential Diagnoses
Mineralization—Oral Cavity

Not applicable.

Other Term(s)Calcinosis circumscripta
Tumoral calcinosis
Ectopic mineralization

Pathogenesis/Cell of Origin
The pathogenesis of calcinosis circumscripta is not known, anda variety of causes have been discussed.

Comment
In the dog, calcinosis circumscripta represents an entity
described in text books as a primarily subcutaneous change;
it is also observed periarticular and has been described in the
tongue of dogs. The lesion is characterized by sharply defined,
irregular areas of amorphous, calcified (Kossa positive) material, surrounded by epithelioid and multinucleated18

Toxicologic Pathology 49(1)macrophages, and peripheral fibrosis. Predisposed are young
dogs and large breeds, in particular German shepherd.24,26,27
The use of the term is not recommended, rather the term“mineralization” should be used instead, and, if consideredappropriate, a comment could be added indicating that thefinding is consistent with calcinosis circumscripta.

Basophilic intranuclear inclusions in the outer spinosum
layers.
Regressing lesions show marked lymphoid infiltrates (Tcells).

Differential Diagnoses
Not applicable.

Eosinophilic Granuloma—Oral Cavity
Pathogenesis/Cell of Origin

Comment

Eosinophilic granuloma (EG) is suspected to be due to an
inherited defect in eosinophil regulation.

Typically, a lesion of young dogs. Outbreaks of papillomatosis
can occur in experimental dog colonies. The papillomas regress
spontaneously after 1 to 2 months, and the dog is protectedfrom reinfection through antibody-mediated immunity.28,29

Diagnostic Features






Diffuse dermal eosinophilic inflammation.
Foci of degranulating eosinophils.
Mast cells and epithelioid macrophages may be present.
Eosinophilic cell debris surrounding collagen fibers(flame figures).
Acanthosis or ulceration of the overlying epithelium.

Differential Diagnoses
Oral mast cell tumor: Anaplastic mast cells and eosinophils are
diffusely present, whereas in the EG, there is a predominanceof eosinophils with a few normal mast cells.

Comment
Eosinophilic granulomas are rare in dogs, the features are similar to those observed in cats. Eosinophilic granuloma may beobserved at all ages, is more common in dogs younger than 3
years old and in males, and has been noted in various breeds;
there is a predilection in Siberian Huskies and cavalier KingCharles spaniels.24,25

Papilloma, Squamous Cell—Oral Cavity(Figures 4.1 and 4.2)
Other Term(s)Oral papillomatosis.

StomachUnlike the rodent, the stomach of the dog consists of glandular
mucosa only and is devoid of a nonglandular mucosa; therefore, the terminology for the nonglandular stomach of rodents
is not applicable for the dog and was omitted. As in otherspecies, the glandular mucosa of the dog is composed of various glands that are present in specific regions of the organ. To
assure a thorough examination of the stomach, examination ofrepresentative sections of all regions is mandatory. In the dog,these include the cardia, fundus, and pylorus regions with correspondingly named glands. The recommended nomenclaturefor nonproliferative and nonneoplastic proliferative lesions inthe stomach of beagle dogs used in toxicity studies are listed
Table 4.2.

Cellularity Increased, Lymphocyte—
Stomach (Figure 4.3)
Other Term(s)Hyperplasia, lymphoid follicles
Hyperplasia, lymphocyte

Pathogenesis/Cell of Origin
Increased number of lymphocytes and plasma cells.

Pathogenesis/Cell of Origin
Hyperplastic response to viral infection; canine papillomavirus 1.

Diagnostic Features

Diagnostic FeaturesVerrucous lesion with a thick squamous epithelium anda fibrous stalk.
Marked acanthosis.
Degeneration of epithelial cells in the stratum granulosum and spinosum with clear cytoplasm and condensed
nucleus.



Increased size of lymphoid follicles, due to increased
number of B- and/or T-lymphocytes, plasma cells, andmacrophages.
Increased number of lymphoid follicles.
Increase in the number of lymphocytes.

Differential Diagnoses
Not applicable.

Woicke et al.

19

Table 4.2. Microscopic Findings in the Digestive System: Stomach; Dog.
StomachCongenital
Ectopic tissuea
Nonneoplastic
Amyloid
Atrophy
Apoptosisa,bApoptosis/single-cell necrosisa,bCellularity increased, lymphocyteb,cCyst, glandular
Dilatation, glands
Edemaa,c
Eosinophilic globules
Erosion/ulcerb
Helicobacter
Hemorrhageb
Hypertrophy, mucous cell
Infiltrate, [insert appropriate cell type]aInflammationa,cMineralizationa
Necrosis, mucosaa,bSecretory depletion
Vacuolation, epitheliuma
Yeast
Proliferative (nonneoplastic)
Diverticulum
Hyperplasia, focal, mucosac
Hyperplasia, diffuse, mucosa
Hyperplasia, neuroendocrine cell
Hypertrophy, chronic pylorusc
Polyp, hyperplastic (regenerative)c
Polyp, inflammatory (benign lymphoid)c

CommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

Terminology addressed in the General/Systemic Pathology section.
Finding more frequent as an induced change.
cTerminology with diagnostic criteria and/or comments described in the text.
b

Comment

Diagnostic Features

Increased lymphoid cellularity, increased size of lymphoidnodules, or increased number of lymphoid nodules are rarefindings in untreated healthy dogs but occur more frequently
in diseased animals and also may be seen frequently as an
induced change. It is considered a benign, nonneoplastic, reactive response to an immune stimulus. If the change pertains tolymph follicles, the sublocation may be added.

Swelling of the stomach wall, without increase in inflammatory
cell infiltrates or fibrosis.

Differential DiagnosesInflammation. Presence of inflammatory infiltrates.
Fibrosis. Increase in collagenous or elastic fibres.

Comment
Edema—Stomach (Figure 4.4)
Other Term(s)In the dog, edema of the stomach wall as the only lesion is rare
and, if noticed, may be due to incidental intoxication,30 whichis unlikely to occur in a laboratory setting.

Not applicable.

Pathogenesis/Cell of Origin
Increased permeability of the vascular system leading to excessive accumulation of vascular fluid in tissues (see also General
Pathology section).

Inflammation—Stomach
Other Term(s)Lymphoplasmacytic gastritis
Scirrhous eosinophilic gastritis

20

Toxicologic Pathology 49(1)Comment

Comment

Variants of inflammation include lymphoplasmacytic gastritis
and scirrhous eosinophilic gastritis.
Lymphoplasmacytic gastritis is characterized by a thickening of the stomach wall by lymphocytes and plasma cells infiltrating area of fibrosis. In their investigation of accompanying
gastric lesions in dogs with intestinal abnormalities, Lidbury
et al31 found that lymphoplasmacytic gastritis was the mostprevalent entity. Scirrhous eosinophilic gastritis is a rare condition in the dog with a focal or multifocal distribution. A
concomitant arteritis has been described; an allergic reaction
or hypersensitivity was suspected.32

Generally, the change is rare and breed dispositions
have been reported, for example, Basenji and Drentse
Patrijshond dog. 24,34 There are anecdotal case reports
for other breeds and species. The diffuse variant is
considered to be analogous to Menetrier disease in
humans.

Polyp, Hyperplastic (Regenerative)—
StomachPathogenesis/Cell of Origin

Hyperplasia, Focal, Mucosa—Stomach(Figure 4.5)

Gastric epithelium.

Other Term(s)Diagnostic Features

Stenosis, pyloric.

Comment

In old dogs, an acquired hyperplastic lesion has been describedas pyloric stenosis, characterized by annular thickening of thepyloric region, due to hyperplasia of the glands, granulation
tissue in the submucosa, and hypertrophy of the lamina
muscularis.24
In toxicity studies in dogs, hyperplasia of the pylorus region
may be observed as a treatment-related finding.33

Hypertrophy, Chronic, Pylorus—Stomach
Other Term(s)Giant rugal hypertrophy, giant hypertrophic gastritis, chronic
hypertrophic gastritis.

Epithelial lesion that protrudes above the mucosal
surface.
Thickened cystic epithelium with a fibrous stalk.
Variable histiocytic or lymphoplasmacytic
inflammation.
Epithelial cells are similar to those of the adjacent normal tissue, no dysplasia.
Nuclei are of normal size and staining intensity.
Mitotic rate may be slightly increased; mitoses in thebase of crypt.

Differential DiagnosesPathogenesis/Cell of Origin

Inflammatory polyp: Normal epithelium, covering granulation tissue with various inflammatory infiltrates orlymphocytic foci.
Adenomatous polyp: Tubular or papillary growth, basophilia, loss of nuclear polarity, increased mitotic rate
along the whole gland.

Gastric glands.

Comment
Diagnostic Features








Thickening of large areas of the gastric mucosa byhypertrophic/hyperplastic glands; grossly resembling
cerebral gyri.
Lesion may be focal or diffuse.
Secondary folds of muscularis mucosae and submucosa
may be present.
Foveolar and glandular hyperplasia.
Progressive loss of parietal cells, replaced by mucous
cells.
Cystic dilatation of mucous glands.

Differential DiagnosesParietal cell hyperplasia, in association with Zollinger-Ellison
syndrome.

Most commonly, canine stomach polyps are observed in thepylorus region.

Polyp, Inflammatory—Stomach
Other Term(s)Benign lymphoid polyp.

Diagnostic FeaturesNormal gastric epithelium
Epithelium covers granulation tissue
Variable inflammatory infiltrates or lymphoid foci withwell-differentiated germinal centers.

Woicke et al.

21

Table 4.3. Microscopic Findings in the Digestive System: Small and Large Intestines; Dog.
Small and large intestines
Congenital
Cyst, squamous
Ectopic tissuea,bMalformation
Nonneoplastic
Amyloid
Apoptosisb,cApoptosis/single-cell necrosisb,cAtrophy
Atrophy, Brunner’s gland
Congestionb
Degeneration/necrosis
Degeneration/necrosis, Brunner’s glands
Degeneration, neuron, myenteric plexus
Dilatation, crypta
Dilatation
Erosion/ulcerc
Hemorrhageb
Hypertrophy
Hypertrophy, Paneth cell
Infiltrate [insert appropriate cell type]b
Inflammationb
Intussusception
Lymphangiectasia
Metaplasia, osseous
Metaplasia, Paneth cell
Mineralizationb
Necrosis, mucosab
Paneth cell, decreased
Parasite
Syncytia, epithelium
Vacuolation, mucosab
Proliferative (nonneoplastic)
Hyperplasia, mucosa
Hyperplasia, Brunner’s glands
Metaplasia, squamous cell

CommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

Terminology with diagnostic criteria and/or comments described in the text.
Terminology addressed in the General/Systemic Pathology section.
cFinding more frequent as an induced change.
bDifferential Diagnoses

Hyperplastic polyp: Thickened cystic epithelium with afibrous stalk.

Comment
As per Head et al,24 examples of inflammatory polyps havebeen described in the dog; in a retrospective analysis, Taulescu
et al35 report 3 cases of inflammatory polyps.

cecum of dogs are smaller and shorter than the rodent cecum.
The ileum communicates only with the colon, and the ileocolic
valve is an apparent structure when this region is opened at thegross examination.
Table 4.3 summarizes the suggested nomenclature of nonproliferative and nonneoplastic proliferative lesions in thesmall and large intestine of the dog.

Ectopic Tissue—Small Intestines
Modifier
Fundic gland, pancreas.

Small and Large Intestine (Duodenum, Jejunum,
Ileum, Cecum, Colon, Rectum)

Pathogenesis

The morphology of the canine intestine is basically similar tothat of rodents and humans, but relative size and length of the

Presence of pancreatic cells or fundic glands in the mucosa or
submucosa.

22

Toxicologic Pathology 49(1)Diagnostic Features
Pancreatic acini with or without islets of Langerhans, in themucosa or submucosa of intestine, adjacent to the mesentericattachment.

Comment
Presence of ectopic gastric mucosa with feature of the gastricfundic gland in the small intestine, in addition to ectopic pancreas as described in the rodent manuscript, is reported in Beagle dogs.36

Diagnostic FeaturesIschemic necrosis
Inflammation
Squamous metaplasia

Differential Diagnoses
Not applicable.

Comment
The condition occurs in small breeds (terriers), preferably inthe submandibular gland.

Dilatation, Crypt—Small and Large Intestine
Other Term(s)

Cyst, mucosa
Dilation, gland

Pathogenesis/Cell of Origin

Metaplasia, Squamous cell—Salivary Glands
Other Term(s)Squamous metaplasia; ductal squamous metaplasia.

Unknown.

Pathogenesis/Cell of Origin
Diagnostic FeaturesMarked dilatation of crypt showing oval, round, pearshaped, or elongated shape.
Lined by intestinal epithelium including goblet cells.
Lumen may contain mucus and cellular and nuclear
debris, but inflammatory cells are few.

Squamous transformation of ductal or acinar epithelium due tovitamin A deficiency.

Diagnostic Features
Squamous epithelium replaces the cubic/cylindrical epitheliumof the salivary gland acini and ducts.

Differential Diagnoses
Diverticulum: Extension of glands through muscularis mucosae into submucosa.

Differential Diagnoses
Not applicable.

Comment
Mucosal cysts appear more common in duodenum and jejunum
than large intestine and are supposed to be acquired as part ofdegenerative process.37

Comment

Salivary Glands

Exocrine Pancreas

Table 4.4 summarizes the suggested nomenclature for nonproliferative and nonneoplastic proliferative changes of the salivary glands.

Table 4.5 summarizes the suggested nomenclature for nonproliferative and nonneoplastic proliferative lesions in the exocrine pancreas of dogs.

Metaplasia, Necrotizing—Salivary Glands
Other Term(s)Necrotizing sialometaplasia, salivary gland infarction.38,39

The condition occurs in small breeds.

Proliferation, Extra-Islet Endocrine Cell
(Nesidioblastosis)—Pancreas (Figures 5.10–
5.13)
Comment

Pathogenesis/Cell of Origin
Trauma-induced compromise of blood vessels; immunemediated vascular damage has been discussed.

Nonneoplastic proliferation of islet and ductular tissue.40 For a
more detailed description, refer to the section on endocrine
pancreas in endocrine system.

Woicke et al.

23

Table 4.4. Microscopic Findings in the Digestive System: Salivary Glands; Dog.
Salivary glands
Congenital
Ectopic tissuea
Nonneoplastic
Accumulation, adipocytes
Amyloid
Apoptosisa,bApoptosis/single-cell necrosisa,bAtrophyCalculus, ductular
Depletion, secretory, acinar cell
Ectasia, duct
EdemaaFibrosisa
Focus, hypertrophic, basophilic
Granules increased, granular duct
Hemorrhagea
Hypertrophy, acinar cell
Infiltrate [insert appropriate cell type]aInflammationa
Metaplasia, acinar cell
Metaplasia, necrotizingc
Mineralizationa
Necrosisa
Pigmenta
Secretory decreased, acinar cell
Secretory decreased, granular duct
Vacuolationa
Proliferative (nonneoplastic)
Hyperplasia
Metaplasia, squamous cellc

CommonUncommonNot observed but potentially relevant

Not Applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

Terminology addressed in the General/Systemic Pathology section.
Finding more frequent as an induced change.
cTerminology with diagnostic criteria and/or comments described in the text.
b

5. Endocrine System—Pituitary, Pineal
Gland, Thyroid Gland, Parathyroid Gland,
Adrenal Cortex and Medulla, Endocrine
Pancreas
Introduction
This chapter provides a recommended nomenclature for classifying microscopic lesions observed in the endocrine system of
the laboratory beagle dog in toxicity studies. The endocrine
system is divided into the pituitary, pineal, thyroid, parathyroid,
adrenal cortex and medulla and the endocrine pancreas (islets ofLangerhans). For detailed general considerations of the endocrine system, refer to the INHAND publication on the rodent.

Pituitary GlandThe pituitary gland (hypophysis) occupies a bony recess (sella
turcica) in the basisphenoid bone and is attached to the
hypothalamus by the infundibular stalk. The pituitary gland has
2 major compartments including the (1) adenohypophysis(anterior lobe) composed of the pars distalis, pars tuberalis,and pars intermedia and (2) neurohypophysis (posterior lobe)
composed of the pars nervosa infundibulum and lobus nervosus. Macroscopically, the adenohypophysis is vascular andsoft, whereas the neurohypophysis is pale with the texture ofbrain tissue.
The adenohypophysis consists of 3 types of endocrine cells
identified by routine H&E staining: acidophils, basophils, andchromophobic cells. The frequency of each cell type varies
according to the age, sex, and physiological and pathologicalstates. The acidophils have a generally uniform pattern of distribution with increased numbers near the central region andproduce growth hormone and/or prolactin. The basophils occur
in greater numbers at the periphery of the adenohypophysis andproduce adrenocorticotropic hormone (ACTH), thyroidstimulating hormone, luteinizing hormone, or folliclestimulating hormone. The chromophobic cells appear inmoderate numbers near the central region and produce ACTH.
The pituitary also contains supporting cells (folliculostellate
cells) and undifferentiated stem cells. The pars tuberalis is composed of epithelioid cells, sinusoids, and occasionally small follicles lined by folliculostellate or endocrine cells. The parsintermedia is located adjacent to the neurohypophysis and is24

Toxicologic Pathology 49(1)Table 4.5. Microscopic Findings in the Digestive System: Pancreas; Dog.
Pancreas
Congenital
Ectopic tissuea
Nonneoplastic
Accumulation, adipocytes
Amyloida
Apoptosisa,bApoptosis/single-cell necrosisb
Atrophy, acinar cell
Autophagic vacuoles, acinar cell
Ectasia, duct
EdemaaEosinophilic globules
Fibrosisa
Focus, basophilic
Halos, peri-insular, decreased
Halos, peri-insular, increased
Hemorrhagea
Hypertrophy, acinar cell
Infiltrate [insert appropriate cell type]aInflammationa
Inflammation, vessel
Metaplasia, ductule
Metaplasia, hepatocytic
Mineralizationa
Necrosisa
Pigmenta
Secretion decreased, acinar cell
Vacuolation, acinar cell
Proliferative (nonneoplastic)
Hyperplasia, acinar cell
Hyperplasia, ductal cell
Proliferation, extra-islet endocrine cell (nesidioblastosis)c

Common Uncommon Not observed but potentially relevant Not applicable
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

Terminology addressed in the General/Systemic Pathology section.
Finding more frequent as an induced change.
cTerminology with diagnostic criteria and/or comments described in the text.
b

separated from it by a fine layer of vascularized pial connective
tissue. The majority of the cells in the pars intermedia are chromophobic cells which produce both ACTH and melanocytestimulating hormone. Immunohistochemistry techniques are
required to accurately identify the specific cell populations.
The neurohypophysis is joined to the hypothalamus by the
infundibular stalk and is composed of nonmyelinated axons andcapillaries supported by modified glial cells (pituicytes). The
capillaries in the pars nervosa are termination sites for unmyelinated axons, which originate from the hypothalamic neurosecretory neurons. Axons arising from supraoptic and paraventricular
nuclei terminate in the pars nervosa. Both oxytocin and vasopressin (antidiuretic hormone) are synthesized in supraoptic and paraventricular nuclei as large precursor molecules, which containboth active hormones and their associated neurophysins. As thebiosynthetic precursor molecules travel along the axons in secretion granules from the neurosecretory neurons, the precursors arecleaved into the active hormones.
The recommended terminology of microscopic lesions
observed in the pineal gland of dogs is presented in Table 5.1.

Extramedullary Hematopoiesis: Pituitary
Gland (Figure 5.1)
Other Term(s)Hematopoietic cell proliferation; erythropoiesis; extramedullary erythropoiesis.

Pathogenesis/Cell of Origin
Extramedullary hematopoietic cells.

Diagnostic features

Small clusters of hematopoietic cells are randomly distributed in the interstitium of the pars distalis.
Usually consist of erythroid cells, but granulocytic precursors and/or megakaryocytes are occasionally present.
Not associated with the parenchymal necrosis ordegeneration.

Woicke et al.

25

Table 5.1. Microscopic Findings in the Endocrine System: Pituitary Gland; Dog.
Pituitary gland
Congenital
Aberrant craniopharyngeal structures
Aplasia/hypoplasia
Persistent Rathke’s pouch
Nonneoplastic
Angiectasis
Atrophy
Cyst
Extramedullary hematopoiesisa,bFibrosisb
Gliosis, pars nervosa
Hemorrhageb
Hypertrophy, pars distalis
Hypertrophy, pars intermedia
Infiltrate [insert appropriate cell type]b
Inflammationb
Metaplasia, osseous
Pigmentb
Pseudocyst
Thrombusb
Vacuolationb
Proliferative (nonneoplastic)
Hyperplasia, pars distalis
Hyperplasia, pars intermediaabCommonUncommonNot observed but potentially relevant

Not applicable
x

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

Terminology with diagnostic criteria and/or comments described in the text.
Terminology addressed in the General Pathology section.

Differential DiagnosesMononuclear cell infiltrates; lymphocytes and histiocytic cells present alone or in addition to mature myeloid
cells.
Focal inflammation: infiltration of leukocytes or lymphocytes associated with cellular necrosis or degeneration.

Pineal GlandThe pineal gland is located on the dorsal midline of the diencephalon and is part of the epithalamus. The gland forms thecaudal boundary of the roof of the third ventricle. The pineal
gland is mainly composed of epithelioid pinealocytes, neurons,
and supporting glial cells. Pinealocytes produce and secrete
melatonin and also contain serotonin. The neurons connect tothe central nervous system, and the gland is innervated by the
sympathetic system. The glial cells are characterized by numerous filaments, and their processes form a glial layer bordering
connective tissue spaces. Myelinated and unmyelinated nerve
fibers are also present.
The recommended terminology of microscopic lesions
observed in the pineal gland of dogs is presented in Table 5.2.

Thyroid GlandThe thyroid gland originates as a thickened plate of epitheliumin the ventral oropharynx to form the thyroglossal duct, which
extends along the midline to the region of the larynx in thefetus. The paired lobes of the thyroid gland develop from thethyroglossal duct on either side of the larynx and proximal
trachea. The ultimobranchial bodies fuse with the thyroid anddeliver the C cells (neural crest origin) to each thyroid lobe.
Accessory thyroid tissue, which lacks C-cells, is common in the
dog and may be located anywhere from the larynx to the diaphragm. Thyroglossal duct cysts may be present in the ventral
anterior cervical region and represent a postnatal, persistentportion of the thyroglossal duct.
The major components of the thyroid gland are follicular
cells and C-cells (parafollicular cells). The follicular cells form
follicles, which contain liquid colloid, the stored form of thyroglobulin. These follicular cells are flattened, cuboidal, or
columnar depending on the biosynthetic activity of the gland.
C-cells are present in the wall of the follicle or between follicles and form large clusters in the central region of the thyroid
lobes. The C-cells predominantly synthesize calcitonin andalso smaller quantities of calcitonin gene-related peptide,
somatostatin, and bombesin, among other minor hormones.
The recommended terminology of Microscopic lesions
observed in the thyroid gland of dogs is presented in Table 5.3.

C-Cell Complex—Thyroid Gland (Figures 5.2
and 5.3)
Other Term(s)Special parafollicular cell complex.

26

Toxicologic Pathology 49(1)Table 5.2. Microscopic Findings in the Endocrine System: Pineal Gland; Dog.
Pineal gland

CommonUncommonCongenital
Striated muscle fibers
Nonneoplastic
Fibrosisa
Infiltrate [insert appropriate cell type]aMineralizationa
Vacuolationaa

Not observed but potentially relevant

Not applicable

x
x
x
x
x

Terminology addressed in the General Pathology section.

Table 5.3. Microscopic Findings in the Endocrine System: Thyroid Gland; Dog.
Thyroid gland
Congenital
C-cell complexa
Cyst, ultimobranchial
Ectopic tissue, thymus
Ectopic tissue, thyroid
Persistent thyroglossal duct
Thyroid dysplasia
Nonneoplastic
Accumulation, adipocytea,bAmyloid
Atrophy
Colloid alteration
Cystic follicle
Dilatation, follicle, diffuse
Hypertrophy, follicular cell
Infiltrate [insert appropriate cell type]b
Inflammationb
Mineralizationb
Pigmentb
Proliferative (nonneoplastic)
Hyperplasia, C-cellaHyperplasia, follicular cellabCommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

Terminology with diagnostic criteria and/or comments described in the text.
Terminology addressed in the General Pathology section.

Pathogenesis/Cell of Origin
Remnant of cells of ultimobranchial body origin.

Diagnostic Features






Clusters of C-cells in various developmental stages.
Small numbers of undifferentiated epithelial cells, ducts,
and cysts are present.
Follicular cells in various stages of differentiation form
small follicles with or without colloid.
C-cell complexes can be seen in intra- and extrathyroidal tissue with no fibrous capsule.
Contains follicles in various stages of differentiation
with undifferentiated cells, small follicles, with small
lacunae and some larger follicles.

Differential Diagnoses
C-cell hyperplasia, focal: Focal proliferation of C-cells
and free of other cells including follicular cells and remnants of ultimobranchial bodies.

Special Techniques for Diagnostics
Immunohistochemistry: Positive for vimentin, anti-19Sthyroglobulin, and calcitonin. The immature follicular cells andC cells are positive for vimentin and 19S-thyroglobulin.41–43

Comment
Vimentin is expressed by the immature follicular cells derived
from the ultimobranchial anlage. The vimentin filaments may
participate in thyroglobulin synthesis and folliculogenesis.

Woicke et al.

27

Table 5.4. Microscopic Findings in the Endocrine System: Parathyroid Gland; Dog.
Parathyroid gland
Congenital
Ectopic tissue, cartilagea
Ectopic tissue, parathyroidb
Ectopic tissue, thymusb
Nonneoplastic
AmyloidAngiectasis
Atrophy
Cyst
Fibrosisb
Hypertrophy
Infiltrate [insert appropriate cell type]b
Inflammationb
Multinucleated giant cells
Proliferative (nonneoplastic)
HyperplasiaabCommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x

Terminology with diagnostic criteria and/or comments described in the text.
Terminology addressed in the General Pathology section.

When the follicular cells accumulate colloid in the follicularlumens, the vimentin immunoreactivity disappears. Typicalthyroid follicles have no immunoreactivity for vimentin.
In the dog, the calcitonin-producing C cells are very common and particularly prominent; these cells are regarded as
normal and do not have to be recorded in toxicity studies. The
C cells are frequently noted in the perithyroidal tissue near thehilus of the thyroid gland and along the main branches of the
thyroid artery. C-cell complexes need to be distinguished from
C-cell hyperplasia, which should be diagnosed only if there is asignificant increase in C-cell numbers throughout each thyroid
lobe compared to age-matched controls. To achieve this, boththyroid lobes should be sectioned longitudinally in a consistentmanner.44,45

Accumulation, Adipocyte—Thyroid Gland(Figures 5.4 and 5.5)
Other Term(s)Fat infiltration, fat replacement, infiltration, adipocyte,
lipomatosis.

Pathogenesis/Cell of Origin
Mature adipocytes.

Comment
Slight accumulation of adipocytes is recognized as an incidental or background lesion in healthy dogs frequently observed in
preclinical studies, but usually not recorded. When the parenchymal tissue is severely or almost completely replaced byadipocytes, the observation should be recorded including indicating the severity.

C-Cell Hyperplasia—Thyroid GlandComment
In the dog, the calcitonin-producing C cells are very common
and particularly prominent. They are regarded as normal and,
therefore, do not necessarily have to be recorded in toxicity
studies. They are frequently noted in the perithyroidal tissue
near the hilus of the thyroid gland and along the main branchesof the thyroid artery. C-cell complexes need to be distinguishedfrom C-cell hyperplasia, which should be diagnosed only ifthere is a significant increase in C-cell numbers throughout
each thyroid lobe compared to age-matched controls. To
achieve this, both thyroid lobes should be sectioned longitudinally in a consistent manner.44,45
The difference between focal C-cell hyperplasia and C-celladenoma is subjective and arbitrary. As a general guideline, thesize of an adenoma should exceed several thyroid follicles;
these smaller adenomas will not compress thyroid follicles andnot exhibit a capsule.45

Diagnostic Features

Mature adipose cells present focally or diffusely in thestroma.
No degenerative change is observed.
Number of follicles may be reduced.

Parathyroid GlandThe 4 parathyroid glands are of endodermal origin derived
from the pharyngeal pouches in close association with the primordia of the thymus. The external gland is on the proximal

28

Toxicologic Pathology 49(1)Table 5.5. Microscopic Findings in the Endocrine System: Adrenal Gland; Dog.
Adrenal gland
Congenital
Cyst, cortex
Ectopic tissue, adrenocortical
Nonneoplastic
AmyloidAngiectasis
Atrophy, cortex
Degeneration, cystic
Extramedullary hematopoiesisa
Fibrosisa
Hemorrhagea
Hypertrophy, cortex, diffuse
Hypertrophy, cortex, focal
Infiltrate [insert appropriate cell type]aInflammationa
Metaplasia, osseousa
Mineralizationa
Necrosis
Persistent X-zone
Pigmenta
Hyperplasia/hypertrophy zona glomerulosab
Thrombusa
Vacuolation, cortex, decreased, focala
Vacuolation, cortex, increased, diffusea
Vacuolation, cortex, increased, focala
Proliferative (nonneoplastic)
Hyperplasia, cortex, diffuse
Hyperplasia, cortex, focal
Hyperplasia, medulla, diffuse
Hyperplasia, medulla, focal
Hyperplasia, subcapsular cellabCommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

Terminology addressed in the General Pathology section.
Terminology with diagnostic criteria and/or comments described in the text.

surface of the thyroid lobe, and the internal gland is embedded
within the medial lateral thyroid lobe.
The parathyroid glands contain a single type of endocrine cell,namely, the chief cells. The parenchyma of the parathyroid glands
consists of densely packed, folded, branching cords of polygonal
chief cells separated by a delicate stroma of reticular and collagenfibers with occasional fibrocytes. The cords are usually 1 to 2 cells
thick arranged in a trabecular or rarely an acinar arrangement.
The chief cells synthesize and secrete parathyroid hormone(PTH) that regulates systemic calcium homeostasis. The
amount of cytoplasm reflects the level of activity of the chiefcells. Chief cells with increased synthesis and secretion of PTH
are hypertrophied with increased cytoplasm and reduced eosinophilia or vacuolation of the cytoplasm.
The recommended nomenclature for microscopic lesions
observed in the parathyroid gland of dogs is presented in Table 5.4.

Ectopic Tissue, Cartilage—Parathyroid
Gland (Figures 5.6 and 5.7)
Other Term(s)Not applicable.

Pathogenesis/Cell of Origin
Developmental anomaly.

Diagnostic Features

Mature cartilaginous tissue appears in the interstitium.
No evidence of tissue damage or inflammation.

Differential Diagnoses
Metaplasia: Metaplasia can occur when there is abnormal stimulation of tissue growth, generally due to aberrant woundhealing from persistent toxic insult or constant mechanicaldisruption of the injured area.

Adrenal Gland (Cortex and Medulla)
The adrenal glands are located bilaterally craniomedial to the kidneys. The outline of the left gland is flattened dorsoventrally andoval in the cranial portion and cylindrical in the caudal projection,and that of the right gland has an acute angular bend with its cortex
projecting cranially. The adrenal gland is composed of 2 different
tissues forming the adrenal cortex and medulla. The cortex

Woicke et al.

29

Table 5.6. Microscopic Findings in the Endocrine System: Endocrine Pancreas (Islets of Langerhans); Dog.
Endocrine Pancreas (Islets of Langerhans)Nonneoplastic
Amyloid, islet
Angiectasis, islet
Apoptosis, islet cell
Atrophy, islet cella
Degranulation, islet cell
Fibrosis, isleta
Hemorrhage, isleta
Hypertrophy, islet cell
Infiltrate [insert appropriate cell type]aInflammationa
Metaplasia, hepatocyte
Pigment, isleta
Single-cell necrosisa
Vacuolation, islet cella
Proliferative (nonneoplastic)
Hyperplasia, islet cell
Proliferation, extra-islet endocrine cellbabCommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

Terminology addressed in the General Pathology section.
Terminology with diagnostic criteria and/or comments described in the text.

originates from mesenchymal cells of the coelomic mesoderm.
These cells initially proliferate near the genital ridge and form thefetal cortex and differentiate to form the adult cortex while the fetal
cortex regresses. The medulla arises from neural crest ectoderm
cells, which migrate from their point of origin into the developing
mesodermal mass. The medulla is separated from the cortex by adelicate network of reticular and loose connective tissues.
Cortical zones consist of the zona glomerulosa, zona fasciculata, and zona reticularis. The outermost cortical zone is thezona glomerulosa, and the cells are in arches and clusters
that constitute approximately 25% of the adrenal cortex. This
zone produces aldosterone which regulates blood pressure andextracellular fluid volume by acting at distal and collecting
tubules of the kidney to promote sodium retention and potassium excretion. The zona fasciculata occupies approximately
50% of the cortex, and cells in this zone are arranged in longanastomosing cords or columns, separated by small capillaries.
They are responsible for the secretion of glucocorticoid hormones (eg, cortisol), which have diverse homeostatic actions
on many organ systems including an increase in glucose production, decreased lipogenesis, and immunosuppression inhigh concentrations. The innermost cortical zone is the zona
reticularis, which has a relatively random and loose network ofcells and composes 25% of the cortex and produces sex hormones (androgens and estrogens). The adrenal cortex is dependent on trophic hormones secreted from the pituitary gland.
The adrenal medulla is composed of chromaffin cells, which
produce catecholamine hormones, norepinephrine (noradrenaline), and epinephrine (adrenaline). Epinephrine is involved instress and induces the “fight or flight” response, whereas norepinephrine functions as a major sympathetic neurotransmitter.
The recommended nomenclature for microscopic lesions
observed in the adrenal gland of dogs is presented in Table 5.5.

Hyperplasia/Hypertrophy, Zona
Glomerulosa—Adrenal Cortex (Figures 5.8
and 5.9)
Other Term(s)Hyperplasia, zona glomerulosa; hypertrophy, zona glomerulosa; thickening, zona glomerulosa.

Pathogenesis/Cell of Origin
Response to endocrine stimulation of the zona glomerulosa.

Diagnostic Features

Diffuse increased thickness of the zona glomerulosa due
to increased cell number or hypertrophy of zona glomerulosa cells.
The cells of the zona glomerulosa may contain eosinophilic cytoplasm or may be vacuolated (normal or
increased numbers and size of vacuoles).
Cytoplasm of zona glomerulosa cells will be eosinophilic when there are reduced cytoplasmic lipid droplets.

Differential Diagnoses
Not applicable.

Special Techniques for DiagnosticsMorphometrical analysis to determine the thickness ofzona glomerulosa.
Ultrastructural examination may be useful.

30

Toxicologic Pathology 49(1)Comment
Drug-induced thickening of the zona glomerulosa can occur withincreased aldosterone secretion and increased number of zona glomerulosa cells. Calcium channel blockers have been reported toinduce hypertrophy/hyperplasia of the zona glomerulosa. Increased
thickness of the zona glomerulosa can occur in dogs with pulmonary
hypertension due to heartworm (Dirofilaria immitis) infestation.46

Endocrine Pancreas: Islets of Langerhans
The endocrine pancreas originates from the foregut endoderm,
and the endocrine cells are arranged in small foci of periductular cells or as aggregates called the islets of Langerhans. The
islets distribute throughout the pancreas including the rightlobe, body, and left lobe. The body and left lobe have various
sized islets, while the right lobe has small islets.
There are 3 major endocrine cell types that produce glucagon(a cells), insulin (b cells), and somatostatin (d cells). Small number of endocrine cells produce pancreatic polypeptide (PP of F
cells), ghrelin, gastrin, or substance P (enterochromaffin cells).
These endocrine cells are located in specific regions of the islets.
In the body and left lobe, b-cells are distributed in all parts of theislet, and a-cells are scattered in the center or periphery of the islet
regardless of the size of the islet. Fewer d-cells are present in thecenter or periphery of the islets. In the right lobe, b-cells occupy
the majority of the islet with few scattered d-cells.47
Table 5.6 contains the recommended nomenclature of microscopic lesions observed in the endocrine pancreas of dogs.

Proliferation, Extra-Islet Endocrine Cell—
Endocrine Pancreas: Islets of Langerhans(Figures 5.10–5.13)
Other Term(s)Nesidioblastosis.

Pathogenesis/Cell of Origin
Congenital anomaly or neogenesis of islet cells arising fromductal epithelium.

Diagnostic Features









Nonneoplastic diffuse or disseminated proliferation of
pancreatic islets.
Irregular shaped islets, ductuloinsular complexes, andbudding of endocrine cells.
Associated adenomatosis may be seen.
Various-sized and irregularly outlined islet cell aggregations that are multifocal or diffuse.
Small clusters of endocrine cells are often distributed
around proliferative pancreatic ducts or ductules.
Acinar cells are atrophied, degenerate with vacuolated
cytoplasm, irregular-shaped, and have apoptosis adjacent to this lesion.
Severe cases have small pancreatic organ volume.

Differential DiagnosesIslet cell hyperplasia—Proliferation of islet cells but not
exocrine and ductular cells.
Islet cell adenoma—Focally proliferation of islet cells withcompression of peripheral tissue and/or encapsulation.
Islet cell carcinoma—Local invasion of capsule and acinar tissue and with proliferation of fibrovascular stroma.
Cellular anaplasia and cellular and nuclear pleomorphism are common.

Special Techniques for Diagnostics
Immunohistochemical staining for endocrine cells and ductcells is useful to identify proliferating cells.

Comment
Nesidioblastosis in dogs is a nonneoplastic diffuse or disseminated hyperplasia of pancreatic islets and ductules. In thelesions, the relative numbers of alpha cells is decreased, andthere is no significant change in the number of beta and delta
cells. Canine nesidioblastosis is not usually demonstrated as afunctional disorder of the endocrine pancreas, whereas inhumans, it is characterized by persistent hyperinsulinemic
hypoglycemia due to defective nonneoplastic proliferation ofb-cells and usually occurs in newborns and rarely adults.48–50

6. Hematolymphoid System—General
Terminology, Bone Marrow, Thymus, Spleen,
Lymph Node, and Other Lymphoid Tissues
Introduction
For general considerations on the hematolymphoid system, referto the rodent manuscript. In addition, guidance on the microscopic pathology of the hematolymphoid system is the subject ofan STP Position Paper51 and is also included in a special issue of
Toxicologic Pathology: A Monograph on Histomorphologic
Evaluation of Lymphoid Organs, Volume 34 (5); 2006.
This chapter includes microscopic findings of the hematolymphoid system, namely bone marrow, thymus, lymph node,
spleen, mucosal associated lymphoid tissue (MALT), otherlymphoid structures, that is, tertiary lymphoid structures andserosa-associated lymphoid structures (TLSs and SALSs), andgeneral hematolymphoid changes. The tabulated list is basedon the nonneoplastic changes described in the INHAND manuscript for rodents. Detailed descriptions will focus on findingsthat have unique features in the dog compared with the rodent,
while the reader is referred to the rodent document for all otherfindings. Findings are tabulated with an indication of theirprevalence or applicability to vehicle control beagle dogs usedin nonclinical toxicology studies. While the rodent manuscript
details descriptive, conventional (traditional), and enhanced
nomenclature, it is the opinion of nonrodent working group forthe dog that more descriptive microscopic findings are generally more appropriate in the dog in general toxicology studies.

Woicke et al.

31

Table 6.1. Microscopic Findings in the Hematolymphoid System: General Terms; Dog.
Not observed but
Not
Common Uncommon potentially relevant applicable

Hematolymphoid system: General terms
Congenital
Aplasia/hypoplasia
Nonneoplastic
Abscess
Amyloid
Apoptosis, increased, lymphocytea
Erythrophagocytosisb
Extramedullary hematopoiesis, (EMH)a
Granuloma
Infiltrate [insert appropriate cell type]c
Inflammation (indicate modifier)c
Inflammation, granulomatous
Inflammation, mononuclear cell, lymphocyte, monocyte, eosinophil, mixed cell,
lymphoplasmacytic, or pyogranulomatous
Inflammation, neutrophil
Metaplasia, osseous
Mineralizationa
Necrosis
Pigment, macrophage
Tingible body macrophage, increased
Vacuolation, macrophagea,b,c
Proliferative (nonneoplastic)
Cellularity, increased, mast cell

x
x
x
x
x
x
x
x
x
x
x

x
x
x
x
x
x
x

a

Finding more frequent as an induced change.
Finding addressed in the General Pathology section.
cTerminology with diagnostic criteria and/or comments described in the text.
b

General Terminology

Comment

General terms that can be used across organs and tissues of
the hematolymphoid system are presented Table 6.1.

Erythrophagocytosis is an important physiologic process to recycle
iron from senescent or damaged erythrocytes to extract heme.54 In
diseased animals, the severity of the process may be increased.

Erythrophagocytosis—Bone Marrow, Spleen,
Lymph Nodes (Figures 6.1–6.3)
Other Term(s)Erythrophagia.

Vacuolation, Macrophage—Bone Marrow,
Spleen, Lymph Nodes, Thymus, Other
Lymphoid Tissues (Figures 6.4–6.5)
Other Term(s)Pathogenesis/Cell of Origin
Erythrophagocytosis is the main route of normal and pathophysiological clearance of damaged erythrocytes. Resident bonemarrow macrophages and red pulp macrophages are essential
in this process.52

Diagnostic Features
Phospholipidosis.

Comment
The descriptive (general) term “Vacuolation, macrophages” is thepreferred term over phospholipidosis. The term phospholipidosis
may be used as a comment or modifier but should only be used if
verified by other methods, for example, electron microscopy.55

Ingestion of red blood cells especially by macrophages.

Bone Marrow
Differential Diagnoses

Artifact.
Emperipolesis: Active penetration or engulfment of one cellby another which remains intact,53 whereas phagocytosis
includes the degradation of ingested cells or materials.

In nonclinical toxicity studies, hematology data in conjunction
with microscopic examination of the bone marrow provide anassessment of overall hematopoietic cellularity. If the combined
data are inadequate, then cytologic evaluation of the bone marrowsmears (from rib or sternum) may be considered for assessing the32

Toxicologic Pathology 49(1)Table 6.2. Microscopic Findings in the Hematolymphoid System: Bone Marrow; Dog.
Bone marrow
Nonneoplastic
Angiectasis
Cellularity decreased, adipocytes
Cellularity decreased, bone marrow
Dyshematopoiesis
Emperipolesisa
Fibrosisb
Hypersegmentation, granulocyte
Serous atrophy of fatb
Proliferative (nonneoplastic)
Cellularity increased, adipocytes
Cellularity increased, bone marrow
Cellularity increased, macrophagec
Cellularity increased, mast cell

CommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x

a

Terminology with diagnostic criteria and/or comments described in the text.
Finding addressed in the General Pathology section.
cFinding more frequent as an induced change.
beffect of test articles on the hematopoietic system.51,56 Other techniques, such as flow cytometric evaluation, clonigenic assays, andelectron microscopy, may also be helpful in certain situations.56
In dogs, bone that contains red marrow, preferably rib, sternum, or vertebrae, is sampled for the microscopic evaluation of
the bone marrow. Long bones such as proximal humerus or
femur should be avoided because the diaphyseal marrow cavitiesof these bones may be almost completely replaced by fat.56
The recommended microscopic terminology for the bonemarrow of dogs is presented in Table 6.2.

Emperipolesis—Bone Marrow (Figure 6.6)
Other Term(s)Engulfment of another cell.

Pathogenesis/Cell of Origin
Megakaryocytes.

Diagnostic Features


Active penetration or engulfment of one cell by anotherwhich remains intact.53

Differential DiagnosesPhagocytosis: The process of phagocytosis includes thedegradation of ingested cells or materials.

Comment
Emperipolesis is often noted in megakaryocytes containing
intact neutrophils or other hematopoietic cells. The significance of the finding is unclear as it can be observed in healthy
animals but also associated with disease.

Thymus
The thymus should be trimmed transversely through the middleof the organ after fixation to include both lobes. The cortex andmedulla should be evaluated separately considering the cortex:
medulla ratio average across multiple lobules.
The thymus involutes (shrinks) with age in dogs. The thymus of beagle dogs undergoes progressive involution during
the period from 6 to 23 months of age.57 Therefore, the thymus
weights are of low if any significance. Keratinization of Hassall’s corpuscles is prominent in young dogs as in minipigs andnonhuman primates.
The recommended microscopic terminology for the thymusof dogs is presented in Table 6.3.

SpleenRoutinely, histopathological evaluation of the spleen involves
examination of a single cross-section, which is sufficient toevaluate the red pulp and includes the serosal surface, but white
pulp may be not adequately present. A longitudinal section will
increase the amount of lymphoid tissue (white pulp) available
for assessment.58 The role of the spleen in dogs is primarily astorage organ, whereas in rodents, it is primarily defensive.59
Due to the at times massive barbiturate-related congestion in
euthanatized dogs, spleen weight is of little value in nonclinical
toxicologic pathology studies.
The recommended microscopic terminology for the spleenof dogs is presented in Table 6.4.

Hyaline Material—Spleen, Lymph Nodes(Figure 6.9)
Other Term(s)Hyaline deposits.

Woicke et al.

33

Table 6.3. Microscopic Findings in the Hematolymphoid System: Thymus; Dog.
ThymusCommonNonneoplastic
Apoptosis, increased, lymphocyte
Cellularity decreased, lymphocyte
Corticomedullary ratio, decreased
Corticomedullary ratio, increased
Cyst, epithelial
Ectopic tissue, (specify tissue)a
Ectopic tissue, parathyroida
Ectopic tissue, thymusa
Hypoplasia
Involution, age-related
Loss of corticomedullary distinction
Necrosis, lymphocyteb
Proliferative (nonneoplastic)
Cellularity increased, epithelial cells
Cellularity increased, lymphocyte
Epithelium-free areas, increased
Thymic corpuscles, increasedab

UncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

Finding addressed in the General Pathology section.
Finding more frequent as an induced change.

Table 6.4. Microscopic Findings in the Hematolymphoid System: Spleen; Dog.
SpleenNonneoplastic
White pulp
Apoptosis increased, lymphocytea
Cellularity decreased, white pulp
Hyaline materialb
Red pulp
Angiectasis
Cellularity decreased, red pulp
Congestion
Contraction
Ectopic tissue, spleen
Erythrophagocytosis
FibrosisHemosiderotic plaquesb
Proliferative (nonneoplastic)
White pulp
Cellularity increased, plasma cell, white pulp
Cellularity increased, white pulp
Red pulp
Aggregates, macrophage, increased
Cellularity increased, adipocyte
Cellularity increased, macrophage
Cellularity increased, mast cell
Cellularity increased, mesothelial cell
Cellularity increased, plasma cell, red pulp
Cellularity increased, stromal cell
Extramedullary hematopoiesis, increaseda
Hyperplasia nodularaabCommonUncommonNot observed but potentially relevant

x
x
x
x
x
x
x
x
x
x
x

x
x
x
x
x

Finding more frequent as an induced change.
Terminology with diagnostic criteria and/or comments described in the text.

x
x
x
x
x
x

Not applicable

34

Toxicologic Pathology 49(1)Table 6.5. Microscopic Findings in the Hematolymphoid System: Lymph Node; Dog.
Lymph nodeNonneoplastic
Cortex, paracortex, and medullary cords
Apoptosis, increased, lymphocyte
Cellularity decreased lymphocyte
Hyaline materiala
Sinuses and lymphatics
Dilatation, sinus
Erythrocytes, intrasinusoidal
Fibrosis
Lymphangiectasia
Proliferative (nonneoplastic)
Cortex, paracortex, and medullary cords
Aggregates, increased, macrophage
Cellularity increased, interdigitating dendritic cell
Cellularity increased, lymphocyte
Cellularity increased, plasma cell
Cellularity increased, stromal cell
Hyperplasia, angiomatous
Hypertrophy/hyperplasia high endothelial venules
Sinuses and lymphatics
Cellularity increased, macrophages, intrasinusoidal
Cellularity increased, mast cellaCommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x

x
x
x
x
x
x
x
x
x

Terminology with diagnostic criteria and/or comments described in the text.

Diagnostic Features
Hyalinized material is occasionally seen in the follicles
of lymph nodes and spleen.60

Differential Diagnoses
Amyloid deposition.
Immunoglobulin accumulation, probably mediated byimmune complexes.

Hemosiderotic Plaque—Spleen (Figures 6.7
and 6.8)

Comment
Senile change, no toxicologic significance.

Lymph Node
Typically examined lymph nodes in toxicologic pathology studies include a section of mandibular and mesenteric lymph node,but other nodes may be sampled as indicated by the study design
or target biology. Ideally, a standardized midsagittal section oflymph node should be obtained to include all the lymph node
compartments, which constitute the follicles and interfollicularcortex, the paracortex (or deep cortex), and the medulla, including
medullary cords and medullary sinuses (Table 6.5).61

Other Term(s)Siderotic plaque, siderofibrotic plaque, sidero-calcific plaque,Gamna-Gandy bodies.

Hyaline Material—Spleen, Lymph Nodes(Figure 6.9)
Other Term(s)PathogenesisLikely sequela of prior hemorrhage
Dystrophic change secondary to trauma

Diagnostic Features
Present in the fibrous connective tissue of the capsule, trabeculae, and/or around the hilar vessels and consists chiefly of
bilirubin, hemosiderin, and/or calcium deposits. May not beseen macroscopically in young dogs but are recognized macroscopically in older dogs as yellow or grey encrustations or
nodules.

Hyaline deposits.

Diagnostic Features
Hyalinized material is occasionally seen in the follicles oflymph nodes and spleen.60

Differential Diagnoses
Amyloid deposition.
Immunoglobulin accumulation, probably mediated byimmune complexes.

Woicke et al.

35

Table 6.6. Microscopic Findings in the Hematolymphoid System: Mucosa-Associated Lymphoid Tissue (MALT); Dog.
Mucosa-associated lymphoid tissue

CommonNonneoplastic
Cellularity decreased, lymphocytea
Degeneration, follicle-associated epithelium
Hyaline material
Lymphangiectasia
Proliferative (nonneoplastic)
Aggregates, macrophage
Cellularity increased, lymphocytea
Cellularity increased, macrophagea
Hyperplasia, follicle-associated epithelium
Hyperplasia, goblet cell, follicle-associated epithelium
Hypertrophy/hyperplasia high endothelial venule (HEV)
Metaplasia squamous, follicle-associated epitheliumaUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x

Finding more frequent as an induced change.

Table 6.7. Microscopic Findings in the Hematolymphoid System: Other Lymphoid Tissues (Tertiary Lymphoid Structures and Serosa-AssociatedLymphoid Structures); Dog.
Other lymphoid structures

CommonNonneoplastic
Tertiary lymphoid structures
Proliferative (nonneoplastic)
Increased serosa-associated lymphoid structures

Mucosa-Associated Lymphoid Tissue
The MALT is usually organized into nonencapsulated accumulations or diffuse collections of lymphocytes scatteredalong most mucosal surfaces. The MALT, a normal anatomical structure, is considered a secondary immune tissue, which
initiates immune responses to specific antigens encountered at
or via the mucosal barrier. Following antigen stimulation,
lymphocytes forming MALT may proliferate, as is commonly
noted with inflammation, but test articles or xenobiotics may
also directly stimulate MALT. It would be inappropriate torecord normal or stimulated MALT as mononuclear or lymphocytic infiltrates or inflammation. The most common
examples of MALT are gastrointestinal-associated lymphoidtissue in the stomach, intestines, and gallbladder; bronchialassociated lymphoid tissue in the lung; nasal-associatedlymphoid tissue in the nasal cavity; conjunctiva-associatedlymphoid tissue in the eyelids. Less frequently, MALT may
be encountered in sections of trachea, larynx, urinary bladder, reproductive tract, and the ducts of various exocrine
glands.
Tonsils represent another secondary lymphoid tissue
that may be examined in toxicity studies in dogs. In thedog, 3 tonsils are present, the lingual tonsil, the paired
palatine tonsil, and the pharyngeal tonsil. 62 For further
anatomical and histological details, please refer to anatomical textbooks or the excellent review article by Casteleyn
et al.62

UncommonNot observed but potentially relevant

Not applicablexx

The recommended microscopic terminology for the mucosaassociated lymphoid tissue of dogs is presented in Table 6.6.

Other Lymphoid Tissues (TLSs and SALSs)
For the definition and function of TLSs and SALSs, the reader
is referred to the rodent manuscript. Usually, these structures
are examined microscopically in toxicologic pathology studies
when they are included in the section of the associated tissue(like MALTs).
The recommended microscopic terminology for otherlymphoid tissues of dogs is presented in Table 6.7.

7. Hepatobiliary System—Liver and Gall
BladderIntroduction
The liver is a common target organ for chemical-induced
toxicity in all species, including dogs, because most drug
exposure occurs via the oral route and the liver receives theabsorbed chemical directly via the portal vein. The liver
metabolizes (biotransforms) many of these substances,termed xenobiotics, for excretion as water-soluble compounds. During biotransformation, intermediate compounds
can pose a risk of cellular injury to hepatocytes and biliary
epithelium.

36

Liver
Liver pathology has been covered in detail in the rodent INHAND
document63 as well as in other publications.64,65 This section
provides recommended terminology for the hepatobiliary systemin the laboratory beagle dog during preclinical toxicity studies
with diagnostic criteria and histology images included only fornomenclature where there are distinguishing features of the process that differ between the dog and the rodent.
In all organs and tissues, including those of the hepatobiliary
system, tumors are very rarely observed in toxicologic pathology studies in beagle dogs and, therefore, are generally notincluded in this manuscript. For the nomenclature and diagnostic criteria of the neoplastic changes, refer to the chapter on
hepatobiliary neoplasms in the textbook of Tumors of Domestic Animals.66,67
Anatomy. A dog liver consists of 4 lobes; the left lobe divided
into the left lateral lobe and left medial lobe; the right lobe
divided into the right lateral lobe and right medial lobe; thequadrate lobe; and the caudate lobe divided into the papillary
process and caudate process. The liver is supplied with bloodfrom the portal vein and the hepatic artery.
The extrahepatic bile duct system includes the hepatic, cystic, and common bile ducts and the gallbladder, which is situated between the left medial and the quadrate liver lobes.
Histology. Histology of dog hepatobiliary system is generally
similar to rodents and has been described.63 A distinct histologic feature of the dog liver is a spiral smooth muscle thatsurrounds the hepatic veins starting at the larger central veinsand persisting in sublobular and larger veins until the hepaticvein is formed. This muscle can constrict in response to various
endogenous and xenobiotic mediators and alter blood flowfrom the liver.68,69
Physiology. Physiology of dog hepatobiliary system is generally
similar to rodents and has been described. This functional heterogeneity across the lobule is characterized by greater drug
metabolism via the cytochrome P450 enzymes in the centrilobular regions of the liver. Periportal regions, supplied with themost oxygenated blood, are generally more active in synthetic
activities. The zonation develops as a result of the oxygen gradient across the acinus or the gradient of hormones and nutrients.
The recommended microscopic terminology for the liver ofdogs is presented in Table 7.1.

Congenital Vascular Anomaly—Liver(Figure 7.1)
Other Term(s)Microvascular dysplasia.

Toxicologic Pathology 49(1)Diagnostic FeaturesCongenital portosystemic shunt: Anomalous vascularconnection between portal vein and systemic venous
system, abnormal portal tracts with small or absent portal vein, proliferated arterioles, and hepatocellular
atrophy.
Portal vein hypoplasia: Similar changes in the portal
tract, but no shunt vessel is present. Portal vein hypertension with ascites is possible.

Differential Diagnoses
Portal vein thrombosis.

Comment
Congenital vascular anomalies are occasionally observed andare generally macroscopic findings made at necropsy, except
for primary hypoplasia of the portal vein, which does not
involve a shunt vessel and requires histologic confirmation.70
Affected animals have small livers, and histologic features inthese dogs include portal vein hypoperfusion, small or absent
portal veins, proliferated hepatic arterioles, and hepatocyte
atrophy. Primary hypoplasia of the portal vein resembles portosystemic shunts histologically, but affected animals often
have portal hypertension and resultant ascites. In the absenceof a diagnosed shunt and particularly in the presence of portal
hypertension, the most likely diagnosis is portal vein
hypoplasia.

Ductal plate anomaly—Liver (Figures 7.2
and 7.3)
Other Term(s)Ductal plate malformation, juvenile polycystic disease/congenital hepatic fibrosis, congenital dilatation of the large andsegmental bile ducts (macroscopic observation).

Pathogenesis/Cell of Origin
Persistence or aberrant remodeling of the embryonic ductal
plate.

Diagnostic FeaturesExtensive bands of portal bridging fibrosis containing
numerous small to dilated irregular tortuous bile ducts.
Hypoplastic or absent portal veins, compensatory arteriolar proliferation.
No nodular regeneration, only minimal inflammation.

Differential Diagnoses
Pathogenesis/Cell of Origin
Abnormal development of portal vasculature.

Congenital vascular disorders.
Chronic biliary obstruction or inflammation.

Woicke et al.

37

Special Techniques for Diagnostics

Bile Plug—Liver (Figure 7.4)

Cytokeratin 7 or 19 to identify biliary epithelium.

Other Term(s)Cholestasis, icterus, pigment, pigment deposition.

Comment
This is a rare developmental anomaly in dogs.70,71

Pathogenesis/Cell of Origin
Obstruction of bile flow, hepatocellular injury.

Apoptosis—LiverOther Term(s)Programmed cell death.

Diagnostic Features

Pathogenesis/Cell of Origin


Mitochondrial or intrinsic pathway.
Death receptor or extrinsic pathway.
Perforin/granzyme pathway.

Diagnostic Features






Cytoplasmic and nuclear condensation.
Nuclear fragmentation.
Intact plasma membrane until late in the process.
Lack of inflammation.
Tingible body macrophages may be present.

Differential Diagnoses
Single-cell necrosis: Single-cell necrosis results from acute
cellular injury and is characterized by cell and nuclear swelling, karyolysis, karyorrhexis, karyopyknosis, pale eosinophilic
cytoplasm, cytoplasmic vacuoles, loss of cellular detail, cellular debris, and inflammation.

Green-brown bile pigment, primarily bilirubin, in hepatic bile canaliculi.
Pigment may appear in Kupffer cells (after rupture ofbile canaliculi).

Differential Diagnoses
Other pigments, including hemosiderin, lipofuscin, porphyrin,
hematoxylin, and formaldehyde: A comprehensive discussionof differentiation is included in the rodent manuscript underpigmentation.63
Proof of bile pigment can be achieved by special stains (eg,iodine test or Hall test).

Intranuclear Brick Inclusions—Liver(Figure 7.5)
Other Term(s)Acidophilic crystalline intranuclear inclusions, paracrystalline
intranuclear inclusions.

Pathogenesis/Cell of Origin
Comment
A comprehensive description of the processes involved and a
guidance on which terminology to use is provided by the publication of the INHAND Apoptosis/Necrosis Working Group.72
In brief:








Use necrosis and apoptosis as separate diagnostic terms.
Use modifiers to denote the distribution of necrosis (eg,necrosis, single cell; necrosis, focal; necrosis, diffuse;etc).
Use the combined term apoptosis/single-cell necrosis
when
there is no requirement or need to split the processes, orwhen the nature of cell death cannot be determined withcertainty, or
when both processes are present together.

The diagnosis should be based primarily on the morphological features in H&E-stained sections. When needed, additional, special techniques to identify and characterize
apoptosis can also be used.

The genesis is unknown.

Diagnostic Features






Crystalline, rectangular to rhomboid brightly eosinophilic homogenous bodies.
Size is up to 15 mm long.
Nuclear membrane occasionally distorted by the
inclusions.
No morphological evidence suggesting cytoplasmic
degeneration.
Ultrastructurally, a basic periodic structure is observedin the crystal that appears to be composed of helicalfilaments. No structural abnormality is noted in the cytoplasm, nucleus, or nuclear membrane in the cells containing these crystals.

Differential Diagnoses

Viral inclusion bodies.
Heavy metal inclusions.

38

Toxicologic Pathology 49(1)Special Techniques for Diagnostics
Brick inclusions are positive with acid-fast staining73,74; it canbe confirmed by electron microscopy.

Comments
Brick inclusions are occasionally observed in young beagle
dogs using toxicological studies, although it commonly appears
in older dogs. There are no evidences of biological or pathologic significance. Similar inclusions occur in the proximal tubular epithelial cells in kidneys in dogs. There are no evidences
relating to heavy metal intoxication nor viral infection.

membrane, appearing less densely stained, with finestrands of cytoplasm (accumulation of glycogen).
Increased sinusoidal neutrophil accumulation.
The change appears panlobular with prominence of midlobular and centrilobular zones.

Differential Diagnoses

Storage disease.
Vacuolation: Well-defined, single or multiple roundedvacuole(s).

Inflammation—Liver

Special Techniques for Diagnostics

Other Term(s)Hepatitis.

To detect glycogen, PAS staining with diastase digestion isroutinely applied using tissues which are fixed with alcoholic
fixative or in frozen sections.

Comment

Comment

Diagnostic term “inflammation” should be used carefully. The
term inflammation is the recommended, preferred term overhepatitis or any etiologic terminology; the etiology, when
determined, may be added as a comment or descriptor to thefinding. The type of inflammation is usually added as a modifier and preferably should include the predominant cell type(eg, neutrophilic, lymphoplasmocytic, histiocytic, etc). Indicating the chronicity as a modifier (peracute, acute, subacute,chronic) is acceptable. Traditionally, hepatic inflammatory
responses have been classified as acute, subacute, chronic,
granulomatous, and so on. These terms are somewhat interpretative, lack precise definitions, vary depending upon study
duration, usually do not consist of a singular cell type, anddo not have exclusive pathognomonic features. A more
descriptive approach is recommended and can be qualified
by lesion distribution or use of subclassification and discretionary qualifiers.

Steroid hepatopathy is limited to dogs primarily, although rabbits and cats are reported to have a similar, but less pronounced
response to corticosteroids. Postprandial and toxicityassociated glycogen accumulation have also been describedin rodents and marmosets.75 Endogenous steroid excess is theresult of Cushing disease or syndrome, due to neoplasia of thepars distalis of the pituitary gland or functional neoplasms of
the adrenal cortex. This finding is uncommon as background inlaboratory dogs used in preclinical studies. Administration ofcompounds with direct or indirect corticosteroid activity canalso produce this lesion. If deemed appropriate or necessary,
qualifiers such as increased or decreased may be added or thegrade of rarefaction can be recorded. Because glycogen is frequently evident in hepatocytes, it is typically only recorded if adrug-related effect is suspected.

Rarefaction—Liver (Figure 7.6 and 7.7)
Other Term(s)Cytoplasmic rarefaction, excess glycogen accumulation, ballooning degeneration, corticosteroid hepatopathy.

Pathogenesis/Cell of Origin
Excessive endogenous or exogenous corticosteroid exposure
leads to significant accumulation of cytoplasmic glycogenbeyond that seen in fed dogs, most likely due to a disturbed
balance between glycogenesis and glycogenolysis.75–77

Vacuolation, Hepatocytes—Liver(Figures 7.8–7.10)
Other Term(s)Fatty change, steatosis, lipidosis, lipid accumulation;
phospholipidosis.

Pathogenesis/Cell of Origin
Excessive accumulation of vacuoles (generally lipids) in thecytoplasm

Diagnostic Features

Diagnostic FeaturesEnlarged hepatocytes (ballooned) with a clear cytoplasm and well-defined eosinophilic cytoplasmic

Macrovesicular: Hepatocytes contain a large welldefined single rounded vacuole within each cell.
Nucleus and cytoplasm displaced to the periphery. A
few hepatocytes may contain one or more smaller
vacuoles.

Woicke et al.
Microvesicular: Hepatocytes partially or completely
filled with numerous small lipid vacuoles. Affected
hepatocytes may have a “foamy” appearance. Smallvacuoles do not normally displace the nucleus to theperiphery.

Differential Diagnoses


Cytoplasmic rarefaction.

Special Techniques for Diagnostics
Frozen sections of unfixed or fixed tissue can be stained for fat(oil red O, Sudan IV) and are helpful in recognizing microvesicular lipidosis and in differentiating fat from other substancesthat may accumulate in vacuoles.

39

Ectopic Tissue, Pancreas—Gallbladder(Figure 7.12)
Comment
For a detailed description of the ectopic pancreas tissue in the
gallbladder, refer to the rodent manuscript (Ectopic Tissue,
Pancreas). To the authors’ knowledge, there is only one reference available that describes this lesion in the dog, specificallyin a Siberian Husky.79 A case of ectopic pancreatic tissue in abeagle dog has been brought to the authors’ attention; refer tothe image (Figure 7.12).

Hemorrhage—Gallbladder
Other Term(s)CommentThe descriptive term vacuolation is the preferred term
over the etiologic terminology listed above (Other
Terms).
It is considered good practice to add modifiers suchas lipid or fatty change, if the nature of thevacuoles has been confirmed by special methodology, for example, lipid stain. In the case of lipid
vacuolation, micro- and macrovesicular vacuolation
may be also indicated by modifier or comment;
other qualifiers and comments may be added(eg, location).
It is recommended to diagnose phospholipidosis, characterized by multiple irregular to round clear
membrane-bound cytoplasmic vacuolation, as vacuolation and indicate phospholipidosis as modifier, descriptor, or comment. Verification by electron microscopy
or immunohistochemical (IHC) staining (LAMP-2)78 isrequired (Figure 7.6C).

Adenoma, Hepatocellular—Liver(Figure 7.11)



Bleeding
Extravasation

Pathogenesis/Cell of Origin
Extravasation of red blood cells.

Diagnostic Features
Red blood cells outside of the vascular system in the parenchyma, interstitial space, or cavity of the hollow organ.

Differential Diagnoses
Congestion. Accumulation of blood within the vascular system.

Comment
Hemorrhage, bleeding, extravasation, or the escape of blood fromthe blood vessels may be observed in virtually every organ and tissue
and is microscopically characterized by the extravasal presence oferythrocytes in tissues. For the types of hemorrhages observed, referto the comment under Hemorrhage in the General Pathology section.
The exact location of the hemorrhage (eg, “submucosa”) may be
added as a modifier, locator, or comment, depending on the dataacquisition system and the pathologist’s preference.

Comment
A rare benign tumor in the liver of beagle dogs, which has
been observed in toxicologic pathology studies. The neoplasm is included in the INHAND rodent manuscript of thehepatobiliary system.63 For the diagnostic criteria in dogs ofthis and other neoplastic findings, readers are encouraged torefer the chapter on hepatobiliary neoplasms in the textbook
Tumors of Domestic Animals66 and/or current literature.

Infarct—Gallbladder (Figure 7.13)
Other Term(s)Not applicable.

Pathogenesis/Cell of Origin
Occlusion of the cystic artery.

Gallbladder and Biliary Tree
Table 7.2 summarizes the recommended nomenclature for gall
bladder changes that can be observed in dogs used in preclinical toxicological studies.

Diagnostic FeaturesTransmural coagulative necrosis
Fibrin thrombi

40

Toxicologic Pathology 49(1)Table 7.1. Microscopic Findings in the Hepatobiliary System: Liver; Dog.
Liver
Congenital
Congenital vascular anomalya
Ductal plate anomalya
Hepatodiaphragmatic nodule
Nonneoplastic
AmyloidAngiectasis
Apoptosisa,bAtrophy, hepatocyte
Bile pluga
Congestionb
Crystalsc
Cysts, bile duct
Cytoplasmic alteration
Degeneration, cystic
Degeneration, hydropic
Erythrophagocytosis, Kupffer celld
Extramedullary hematopoiesisb
Fatty change (see Vacuolation)
Fibrosisb
Helicobacter sp. hepatitis
Hypertrophy, hepatocytec
Hypertrophy/hyperplasia, Kupffer cellc
Hypertrophy/karyomegaly, endothelial cell
Inclusions, intranuclear and cytoplasmic
Inclusions, intranuclear brick inclusionsa
Infarct
Infiltrate [insert appropriate cell type]b,c,e
Infiltrate, mixed inflammatory cellb,c,e
Infiltrate, mononuclear cellb,eInfiltrate, neutrophilb,c,e
Infiltrate, peribiliary (intrahepatic)b,c,eInflammationa,bIntrahepatocellular erythrocytes
Intravascular hepatocytes
Karyocytomegaly and/or multinucleated hepatocytesc
Metaplasia, glandular, hepatocyte
Metaplasia, pancreatic acinar cell
Mineralization^
Mouse hepatitis virus hepatitis
Murine norovirus hepatitis
Necrosis, focal/multifocalc
Necrosis, zonal
Phospholipidosisc
Pigment
Rarefactiona
Single-cell necrosisb
Thrombusb
Tyzzer’s disease (Clostridium piliforme infection)
Vacuolation, hepatocytea
Proliferative (nonneoplastic)
Cholangiofibrosis
Focus of cellular alteration
Hyperplasia, bile ductc
Hyperplasia, hemangioendothelial
Hyperplasia, hepatocyte, nonregenerative
Hyperplasia, hepatocyte, regenerativec

CommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
(continued)

Woicke et al.

41

Table 7.1. (continued)
Liver

CommonHyperplasia, Ito cell
Hyperplasia, oval cellc
Neoplastic
Adenoma, hepatocellulara

UncommonNot observed but potentially relevant

Not applicable

x
x
x

a

Terminology with diagnostic criteria and/or comments described in the text.
Terminology addressed in the General Pathology section.
cFinding more frequent as an induced change.
d
For criteria and description of erythrophagocytosis, refer to the Hematolymphoid System/General Terms.
eFor the recommended use of infiltrate, refer to Infiltrate [insert appropriate cell type] in the section General/Multisystemic Pathology.
b

Table 7.2. Microscopic Findings in the Hepatobiliary System: Gallbladder and Biliary Tree; Dog.
Gallbladder and biliary tree

CommonUncommonCongenital
Ectopic tissue, hepatocytes
Ectopic tissue, pancreasa
Nonneoplastic
Calculus
Hemorrhagea
Hyalinosis
Infarcta
Inflammation
Mucocelea
Proliferative (nonneoplastic)
Hyperplasia
Hyperplasia, cystic mucinousa
Metaplasia, glandulara

Not observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x

Terminology with diagnostic criteria and/or comments described in the text.

Differential Diagnoses

Diagnostic Features

Gall bladder mucocele may occur concurrently or infarction
may occur as a single entity.

Special Techniques for Diagnostics



Phosphotungstic acid hematoxylin staining may help identify
arterial thrombi.

Dilatation with accumulation of mucoid secretion.
Mucosal proliferation with projecting epithelial
fronds.
Mucus-filled cysts.
Necrosis and rupture of the wall may ensue.

Differential Diagnoses
Comment80

The pathogenesis of arterial thrombosis is not known.



Cystic mucinous hyperplasia of the gall bladdermucosa.
Extrahepatic biliary obstruction.

Mucocele—Gallbladder (Figure 7.14)
Other Term(s)Not applicable.

Special Techniques for Diagnostics
The PAS may help highlight the mucus in the lumen of the gall
bladder.

Pathogenesis/Cell of Origin
Etiology is unknown, may be related to decreased gallbladdermotility, bile stasis, altered bile composition, and viscosity.

Comment
This condition may cause extrahepatic biliary obstruction.81,82

42

Toxicologic Pathology 49(1)Hyperplasia, Cystic Mucinous—Gall Bladder(Figure 7.15)
Other Term(s)Cystic hyperplasia, cystic mucinous hypertrophy, papillaryadenomatous hypertrophy.

Pathogenesis/Cell of Origin
Unknown.

Diagnostic Features







Macroscopically, gallbladder is enlarged and consistedof thickened wall with numerous multilocular, gelatinous translucent cysts.
Thickened mucosa by numerous fronds and cysts.
Mucosal membrane consists of high columnar epithelial
cells containing and secreting abundant mucinous
materials.
Little or no inflammatory infiltrate.

Differential Diagnoses


Gallbladder mucocele.

Special Techniques for Diagnostics
Not applicable.

Comments
Diagnostic overlap with gallbladder mucocele as some cases
may have elements of cystic mucinous hyperplasia as well
as excessive production of abnormal mucus. Regarded as an
age-related disorder, so very uncommon in the laboratory
beagles generally used in preclinical toxicology studies.
This finding may be associated with exposure to progestational agents.83,84

8. Integumentary System (Skin)
Introduction
This section provides a proposed standardized nomenclature for classifying lesions observed in the skin and appendages of dogs, namely epidermis, dermis, hair follicles,
sebaceous glands, apocrine (epitrichial) glands, eccrine(atrichial) glands, and nailbed using standard H&Estained paraffin sections only. Lesions in the mammary
gland are covered separately. Dermal lesions, which werecovered in the soft tissue, nervous system, or cardiovascular system INHAND rodent manuscripts, are also
covered separately, under those respective systems. The
nomenclature of skin lesions in the dog differs from therodent most strikingly in the classification of neoplasms.
The authors recommend the use of the WHO nomenclaturefor domestic animals or the more recently published text

titled “Tumors of Domestic Animals”1 for reference if askin neoplasm is identified in chemical or pharmacological
toxicity studies.
In this section, only notable differences between dogs
and rodents will be addressed, as the anatomy of dog skinis comparable to that of other mammals. For detailed general considerations on the integument, please refer to the
INHAND publication on rodent integument,85 and, for a
detailed description of the anatomy and physiology of canine
skin, refer to Muller and Kirk’s Small Animal Dermatology.86
Surveys of skin lesions in beagle dogs are included in the
publications of Sato et al49 and Scudamore.60 Parasitic diseases in the dog include flea infestations (Ctenocephalides
canis and others), sarcoptic mange (Sarcoptes scabiei), anddemodectic mange (Demodex canis). For details on the
diagnostic criteria of parasites, the reader is referred to textbooks on veterinary parasitology. In laboratory settings of
toxicologic pathology studies, parasitic diseases of the skinare very rare. When parasites are observed, they should berecorded as “parasite” and, if possible, specified further in acomment to the finding. Also refer to the General/Multisystemic Pathology section.
In general, the thickness of dog skin ranges from 0.5 to 5.0mm, with the thickest areas on the head, dorsum of the neck,back, and sacrum.86 In a recent study, the thickness of beagle
dog skin was referred with 1.3 to 1.4 mm at different measurement sites.87 The epidermis of normal haired skin varies
from 0.1 to 0.5 mm with 2 to 3 nucleated keratinocyte
layers.88 Rete ridges are not found in the normal haired dog
skin but may be observed at the nasal planum, footpads, andscrotum. The epidermal surface varies from undulating at thedensely haired skin of the back to heavily folded at the skin of
the abdomen.88 The surface of footpads is papillated. Besidekeratinocytes, the epidermis comprises several other cell
types, for example, melanocytes, Langerhans cells and Merkel cells.89 The melanocytes are located in the basal layer ofthe epidermis, in the outer root sheath of hair follicles, in thehair matrix, and in ducts of sebaceous and sweat glands. Single melanocytes can also be found in the superficial dermis.
Long dendritic processes of melanocytes extend between keratinocytes and transfer pigment-containing melanosomes tokeratinocytes.86
The canine dermis does not usually have dermal papillae. The dermis is sparsely populated with cells, but there
can be several mast cells around superficial dermal bloodvessels.86
The beagle dog, most commonly used in preclinical safety
studies, has a short coat composed of primary (outercoat) andsecondary (undercoat) hair. Canine hair follicles are arranged
in clusters of several primary hairs surrounded by groups ofsmaller secondary hairs. Each primary hair is accompanied by
sebaceous glands, apocrine sweat glands, and arrector pili muscles, whereas secondary hairs may have sebaceous glands.90
Sebaceous glands and apocrine sweat glands are distributed
throughout the haired skin, whereas eccrine sweat glands arelimited to the footpads. Besides these adnexal glands, dogs

Woicke et al.

43

Table 8.1. Microscopic Findings in the Integument: Skin; Dog.
Skin
Nonneoplastic
Skin: Epidermis
Apoptosisa,b,c
Atrophy, epidermis
Edema, epidermis, intercellular
Edema, epidermis, intracellular
Erosion/ulcer
Hyperkeratosis, epidermis
Infiltrate [insert appropriate cell type], epidermisb
Necrosis, epidermis
Parasiteb.d
Pigment, decreaseda
Pigment, increaseda
PustuleVesicleSkin: Cutaneous adnexa
Adnexal dysplasia
Atrophy, adnexal
Hyperkeratosis, adnexa
Inflammation, adnexab
Necrosis, adnexac
Parasiteb,dSkin: Dermis and subcutis
Atrophy, dermis
Edema, dermis
Elastosis
Parasiteb,dProliferative (nonneoplastic)
Skin: epidermis
Cyst, squamous
Hyperplasia, epidermisa
Papilloma, squamousa
Skin: Cutaneous adnexa
Cysta
Hyperplasia, adnexaa
Skin: Dermis and subcutis
Hyperplasia, melanocyte
Polyp, fibroepitheliala
Neoplastic
Cutaneous histiocytomaa,eCommonUncommonNot observed but potentially relevant

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

x
x
x
x
x
x
x
x

a

Terminology with diagnostic criteria and/or comments described in the text.
Terminology addressed in the General (Multisystemic) Pathology section.
cFinding more frequent as an induced change.
d
For the recording of parasites, refer to the General (Multisystemic) Pathology section.
e
The cutaneous histiocytoma is also included in the soft tissues section.
b

have specialized glands including circumanal glands, glands ofexternal ear canal, the anal sac gland, and the tail gland. Circumanal glands and tail gland (supracaudal gland) are composedof hepatoid cells, which can also be found in the prepuce. Analsac glands are composed of abundant large sebaceous glands andapocrine sweat glands, whereas the glands of the external ear
canal (ceruminous glands) are apocrine glands. Meibomian
glands are modified sebaceous glands.86
Table 8.1 summarizes the microscopic lesions that can beobserved in the skin of beagle dogs in toxicological safety
studies.

Apoptosis—Skin
Other Term(s)Programmed cell death.

Pathogenesis/Cell of Origin


Mitochondrial or intrinsic pathway
Death receptor or extrinsic pathway
Perforin/granzyme pathway

Not applicable44

Toxicologic Pathology 49(1)Diagnostic Features






Cytoplasmic and nuclear condensation
Nuclear fragmentation
Intact plasma membrane until late in the process
Lack of inflammation
Tingible body macrophages may be present

associated with hydropic degeneration of basal cells and subsequent hypopigmentation, the underlying superficial dermis
usually reveals pigmentary incontinence characterized byincreased size and/or number of melanosomes. The mechanismof pigmentary incontinence is unclear but could include epidermal injury and melanin release to the superficial dermis
through a damaged basement membrane.

Differential Diagnoses
Single-cell necrosis: Single-cell necrosis results from acute
cellular injury and is characterized by cell and nuclear swelling, karyolysis, karyorrhexis, karyopyknosis, pale eosinophilic
cytoplasm, cytoplasmic vacuoles, loss of cellular detail, cellular debris, and inflammation.

Comment
A comprehensive description of the processes involved anda guidance, which terminology to use is provided by the publication of the INHAND Apoptosis/Necrosis Working Group72.
In brief:







Use necrosis and apoptosis as separate diagnostic terms.
Use modifiers to denote the distribution of necrosis (eg,necrosis, single cell; necrosis, focal; necrosis, diffuse;etc.).
Use the combined term apoptosis/single-cell necrosiswhena. there is no requirement or need to split the processes, orb. when the nature of cell death cannot be determinedwith certainty, orc. when both processes are present together.
The diagnosis should be based primarily on the morphological features in H&E-stained sections. When needed,
additional, special techniques to identify and characterize apoptosis can also be used.

Diagnostic Features

Decreased or absent pigment in a normally pigmented
epidermis.
Melanin clumps and/or increased size and/or number ofmelanophages in the underlying dermis (pigmentary
incontinence).

Comments
Decreased pigmentation may be associated with pigmentary
incontinence. In general, pigmentary incontinence is notrecorded separately in toxicity studies, but may be described
as a component of a distinct lesion with melanin release to thesuperficial dermis.

Pigment Increased—Skin
Other Term(s)Hyperpigmentation, melanosis.

Pathogenesis/Cell of Origin
Increased deposition of melanin within the epidermis. Skin
pigmentation is affected by local inflammation because melanocytes respond to inflammatory mediators by increasing or
decreasing melanogenesis and by altering melanin transfer tokeratinocytes.

Diagnostic Features
Pigment Decreased—Skin
Other Term(s)Hypopigmentation, hypomelanosis.

Pathogenesis/Cell of Origin
Decreased pigmentation reflects lower amounts of melaninpigment in the epidermis either microscopically or macroscopically (loss of skin or hair pigmentation). While it may be
associated with congenital or acquired idiopathic defects on
melanization in some dog breeds, in toxicologic pathology,
most often it is the result of the effects of certain chemicals(eg, monobenzyl ether of dihydroquinone) on melanocytes,
inflammatory disorders that affect melanization or destroy melanocytes, hormonal disorders, or dermatoses featuring hydropic degeneration of basal keratinocytes. In dermatoses

Deposition of excessive amounts of melanin within theepidermis.
It may be confined to the stratum basale or present
throughout all epidermal layers.
Increased melanin may be present within keratinocytes
and/or dermis (within melanophages).

Comments
Hyperpigmentation is a common, nonspecific finding that can beobserved in many skin diseases associated with chronic inflammation, numerous hormonal dermatoses, as well as some developmental and neoplastic disorders. In general, this finding is notdescribed or recorded separately in toxicity studies but may bedescribed as a component of a complex inflammatory finding or
may be recorded when describing a chronic or pronounced
change. Increased pigment may be observed in the hormone

Woicke et al.
responsive skin of dogs that have been administered the dopaminergic and prolactin-inhibiting agent, bromocriptine.91,92

45

Differential DiagnosesHyperplasia, Epidermis—Skin
Comment
Epithelial hamartoma (pigmented epidermal nevus, linear epidermal hamartoma, verrucous epidermal nevus), Cutaneoushorn (papillomatous hyperplasia), and Seborrheic keratosis are
considered entities to be summarized under epidermal hyperplasia. A comment may be added to the diagnosis to further
specify the lesion.
Epidermal hamartomas are congenital or acquired malformations that are commonly recognized only later in life. In thedog, association with papilloma virus has been described incases. Inflamed linear epidermal nevus is a very rare lesionin dogs. The lesion is characterized by focal irregular epidermal papillary hyperplasia, laminated hyperkeratosis, and keratohyalin granules and may be present in the stratum
granulosum and may be hyperpigmented. The inflamed linear
epidermal nevus is characterized by prominent parakeratosis,
neutrophilic pustules, and variable dermal inflammation.
The spontaneous cutaneous horn is a very rare lesion inbeagle dogs. It may occur at any age and has been observedin association with other hyperplastic or neoplastic skin entities. Microscopically, the change is characterized by an exophytic cylindrical formation of compact keratin, focally
hyperplastic epidermis, extensive, compact, laminated, orthoor parakeratotic hyperkeratosis. On the surface, the stratum
corneum forms columns.
Seborrheic keratosis is a benign, acanthotic, and often pigmented epidermal proliferation; other features include an exophytic or slightly endophytic growth pattern, monomorphic andbasaloid appearance of proliferating keratinocytes, and multifocal keratin accumulations that may form small horn cysts,
and hyperpigmentation due to numerous melanin granules distributed throughout proliferating keratinocytes. The lesion israrely diagnosed in the dog. So far, no breed, sex, or site predilection was recognized. The lesion seems to occur more frequently in older dogs

Squamous Papilloma—Skin
Other Term(s)Idiopathic squamous papilloma, warts.

Pathogenesis/Cell of Origin

Cutaneous horn
Cutaneous exophytic papilloma: canine papillomavirus (CPV) induced neoplastic lesion and pedunculated or cauliflower-like circumscribed alopecic
hyperkeratotic mass, observed mainly on the head,eyelids, or paws. Often regresses spontaneously. Histopathologically, proliferating keratinocytes show avariable viral cytopathic effect and intranuclear inclusion bodies.
Cutaneous inverted papilloma: Canine papillomavirus
induced, single or multiple, raised, firm neoplasm witha central keratin-filled pore; cup-shaped appearance.
Occurs most commonly on the abdomen or groin, also
on limbs and digits. Does not regress spontaneously.
Histopathologically, it may be cup shaped with a central
core of keratin. Keratinocytes may show a viral cytopathic effect.

Cyst, Adnexa—Skin
Comment
A number of cystic lesions with hyperplastic properties havebeen described in the skin of dogs. Usually, their terminology
reflects the origin and/or their features. Examples are:











Infundibular cyst (epidermoid cyst, epidermal cyst, epidermal inclusion cyst)
Isthmus cyst
Panfollicular cyst (Trichoepitheliomatous cyst)
Dermoid cysts
Sebaceous duct cyst
Subungual epithelial inclusion cyst
Apocrine cyst (apocrine cystomatosis)
Ceruminous cyst
Ciliated cyst
Comedone, pressure point

These conditions are generally rarely observed in toxicologic
pathology studies. Should these lesions occur, it is recommended
to diagnose them as cyst with a descriptive modifier. For the
diagnostic criteria, refer to the section Epithelial and Melanocytic Tumors of the Skin by Goldschmidt and Goldschmidt93 in the
textbook “Tumors of Domestic Animals”, to “Integumentary
System” by Mauldin and Kennedy27 in Jubb, Kennedy, and Palmer’s Veterinary Pathology, Volume I, the WHO fascicle Histological Classification of Epithelial and Melanocytic Tumors of
the Skin of Domestic Animals,94 and/or pertinent literature.

Originates from squamous epithelium of the epidermis.

Diagnostic FeaturesPapillary mass
Hyperplastic stratified squamous epithelium
Supporting dermal stroma

Hyperplasia, Adnexa—Skin
Comment
Apocrine (sweat gland) hamartomas, fibroadnexal hamartomas, follicular hamartomas, and sebaceous hamartomas are46

Toxicologic Pathology 49(1)considered entities to be summarized under adnexal hyperplasia. The lesions originate from apocrine, follicular, and/or
sebaceous epithelial cells. Microscopically, they are characterized by an accumulation of disorganized but mature specialized
cells or tissues indigenous to the particular site. The lesions
represent proliferative lesions of hair follicles, sebaceous
glands and ducts, apocrine glands, or pilosebaceous units. A
comment may be added to the diagnosis to further specify thelesion.
Apocrine (sweat gland) hamartomas are rare but have been
described in dogs; they are proliferating sweat glands in the
superficial dermis and/or panniculus; apocrine glands arelarger than normal, ectatic and lined by attenuated epithelium;
the overlaying epidermis may be hyperplastic.
Fibroadnexal hamartomas (adnexal nevus, focal adnexal
dysplasia, folliculosebaceous hamartoma) occur most commonly on the distal limbs, pressure points, and in interdigitalareas. The lesions may be congenital or acquired following
trauma to the area. Microscopic features include hyperplastic,
distorted, and dilated hair follicles; no connection to the skinsurface and no hair bulbs; surrounded by dense, mature collagen; and variable inflammation.
Follicular hamartomas (follicular nevus) are rare lesions that
occur more frequently in younger dogs. Predisposed sites
include the limbs and neck. Microscopically, the lesions consistof clusters of very large anagen hair follicles that extend moredeeply than adjacent normal follicles. In the majority of cases,
hair follicles within a cluster are uniform and have the appearance of primary follicles; associated glands may be enlarged.
The lesions are surrounded by hyalinized collagen, and secondary folliculitis may occur.
Sebaceous hamartomas are regarded as an uncommon variant of the fibroadnexal hamartoma characterized by an intradermal proliferation of predominantly large sebaceous glands
and ducts. They are located in the mid to deep dermis without
involvement of superficial dermis; hair follicles are small andmalformed; the lesions are surrounded by mature collagen.

Comment
Fibroepithelial polyps are fibrovascular lesions that rather represent a proliferative response to trauma or inflammation thanactual neoplasms. They occur mostly on the trunk, sternum,
and pressure points of the limbs and may be solitary or
multiple.

Cutaneous histiocytoma—Skin(Figures 8.1–8.3)
Other Term(s)Not applicable.

Pathogenesis/Cell of Origin
Langerhans cells.

Diagnostic Features










Polyp, Fibroepithelial—Skin
Other Term(s)Cutaneous tag, skin tag, acrochordon, fibrovascular papilloma.



Diagnostic Features

Mature dermal collagenous tissue that can be highly
vascularized.
No adnexal structures within the lesion but may be
hyperplastic at the base of the polyp.
Covered by an irregular, hyperplastic, hyperkeratotic,
and hyperpigmented epidermis.

Grossly, a dome-shaped protrusion of the skin, hairless,often abraded, and ulcerated.
Dermal (and epidermal) infiltrate of densely packed,
mildly pleomorphic round cells, arranged in cords andnests.
Histiocytic-like neoplastic cells, ovoid, bean-shaped, or
convoluted nuclei, lightly eosinophilic cytoplasm.
Often numerous mitotic figures.
Typically, wedge shaped, that is, narrow basis and wider
near the epidermis,95 “top heavy” epidermal focus.27,96
Little or no stroma.
Lymphocytic infiltrates at the base of the neoplasm may
be present during regression.
Immunohistochemistry may be used to provide definitive proof97; canine cutaneous histiocytomas are considered a Langerhans cell tumor and express CD1a, MHC
II, CD11c, CD18, and often E-cadherin.96,98

Differential Diagnoses

Pathogenesis/Cell of Origin
Proliferative fibrovascular lesion

Few amounts of inflammatory cells.
Secondary ulceration and neutrophilic inflammation due
to trauma may occur.

Epitheliotropic T-cell lymphoma: When considered
because of epithelial involvement, differentiation is best
done by IHC.
Plasmacytoma: Sheets of round cells, in nests or poorly
defined cords, pleomorphic nuclei, mono, bi-, or multinucleated, no or few mitoses, scant to moderate eosinophilic or basophilic cytoplasm, amyloid may bepresent. The IHC may be required for definite differential diagnosis; MUM1 is considered the most reliable
marker.99
Cutaneous histiocytosis: Coalescence of perivascular
infiltrates in the subcutis and deep dermis without involvement of epidermis.

Woicke et al.

47

Table 9.1. Microscopic Findings in the Mammary Gland; Dog.
Mammary gland
Nonneoplastic
AmyloidAngiectasis
Apoptosis/single-cell necrosisa,bAtrophy
Basophilia
Congestionb
Corpora amylacea
Degeneration
Dilatation
Edemab
Fibrosisb
Hemorrhage^
Hypertrophy/hyperplasia, alveolar and/or ductal cell
Infiltrate [insert appropriate cell type]b
Inflammationb
Mineralizationb
Necrosisb
Parasiteb
Pigmentb
Single-cell necrosis^
Thrombosisb
Proliferative (nonneoplastic)
Hyperplasia, lobuloalveolarabCommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

Terminology with diagnostic criteria and/or comments described in the text.
Terminology addressed in the General Pathology section.

Comment
Usually a solitary lesion of young dogs, less than 3 years old,but has been observed in dogs of all ages. The lesion occurs inall breeds, predisposed are brachycephalic breeds, includingboxers, but also dachshund, Doberman pinschers, and cocker
spaniels. Spontaneous regression is a feature.96,100

9. Mammary GlandIntroduction
For detailed general considerations, refer to the INHAND
rodent publication.101 A detailed description of estrous cyclerelated changes in the canine female genital tract, including the
mammary gland, is provided in a review by Rehm et al102;
cycle-related changes of the mammary gland are also presented
in Table 11.1 of the female reproductive system. The following
findings may be observed as part of the normal estrus cycle indogs: fibrosis in early diestrus, apoptosis (single-cell necrosis),and pigment accumulation in proestrus.103
In routine toxicologic pathology studies, 1 section of the
mammary gland, usually one of the inguinal glands, is examined per female and male. The section should include the nipple, the duct, and a representative amount of glandular tissue.
In male dogs, glandular tissue may be difficult to obtain; thepresence of ductular structures is considered sufficient in thesecases.

Substantial differences between dogs and rodents areobserved in the morphology and terminology of mammary
gland neoplasms; for a detailed description of the neoplastic and tumor-like lesions, refer to the “International Histological Classification of Tumors of Domestic Animals:
Mammary Tumors of the Dog and Cat,” published by the
Armed Forces Institute of Pathology in conjunction with
the American Registry of Pathology and the World HealthOrganization,104 the more recently published “Tumors ofDomestic Animals,”105 and other pertinent literature.
The recommended terminology of microscopic lesions in
the mammary gland of dogs is presented in Table 9.1.

Apoptosis—Mammary Gland
Other Term(s)Programmed cell death.

Pathogenesis/Cell of Origin


Mitochondrial or intrinsic pathway
Death receptor or extrinsic pathway
Perforin/granzyme pathway

Diagnostic Features

Cytoplasmic and nuclear condensation
Nuclear fragmentation

48

Toxicologic Pathology 49(1)Intact plasma membrane until late in the process
Lack of inflammation
Tingible body macrophages may be present

Differential Diagnoses
Single-cell necrosis: Single-cell necrosis results from acute
cellular injury and is characterized by cell and nuclear swelling, karyolysis or karyorrhexis, nuclear pyknosis, pale eosinophilic cytoplasm, cytoplasmic vacuoles, loss of cellular detail,cellular debris, and inflammation.

Comment
A comprehensive description of the processes involved andguidance on which terminology to use is provided by the
publication of the INHAND Apoptosis/Necrosis Working
Group.72 In brief:
1.
2.
3.
4.
5.
6.
7.

Use necrosis and apoptosis as separate diagnostic terms.
Use modifiers to denote the distribution of necrosis (eg,necrosis, focal; necrosis, diffuse, etc).
Use the combined term apoptosis/single-cell necrosis,
when
there is no requirement or need to split the processes, orwhen the nature of cell death cannot be determined withcertainty, or
when both processes are present together.
The diagnosis should be based primarily on the morphological features in H&E-stained sections. Whenneeded, additional, special techniques to identify andcharacterize apoptosis can also be used.

10. Nervous System—Brain, Spinal Cord, andPeripheral Nerve
Introduction
Species used for neurotoxicological research exhibit 3 different
general models of brain architecture based on the prominenceof cerebrocortical subdivisions (archicortex, paleocortex, neocortex). The rodent model, which also applies for the rabbit, ischaracterized by a lissencephalic cerebral surface and large
archicortex. The carnivore (dog, cat) model shows prominent
gyri and sulci in the cerebrum, which abuts the cerebellum but
does not cover it in contrast to the primate model. The carnivore model shows far less complex cerebrocortical subdivisions than the primate. The most striking difference between
carnivore and rodent brain is the presence of a well-developed
neocortex in carnivores. In the spinal cord, a notable difference
between rodent and carnivore is the much larger size of thedorsal funiculus in carnivores. The location and size of spinal
motor tracts show notable species differences: the corticospinal
tract of rats is found in the dorsal funiculus, while in carnivores
and primates, it is located in the lateral funiculus; compared tocarnivores, the rodent shows smaller sensory tracts in the dorsal

funiculus. In the brain stem, the olivary nucleus is enlarged in
carnivores and primates compared to rodents, reflecting also
the greater lateral expansion of the cerebellar hemispheres. The
size of the medial and lateral lemniscus is increased in carnivores and primates compared to rodents. The profile of thevermis on the caudal surface of the cerebellum is generally
straight in rodents and primates but is twisted into a serpentine
loop in carnivores because of its elongation in these species.
The corpus striatum is unified in rodents but divided in carnivores and primates into caudate and lentiform nuclei. The
information summarized here came from a review article,106
and the reader is referred to that publication for additional
details. Other pertinent references include:
A comparative brain atlas of rat, dog, and monkey in Fundamental Neuropathology for Pathologists and Toxicologists,
Chapter 3.107
The atlas “The Beagle brain in stereotactic coordinates”108
is a useful reference for neuroanatomy in the dog.
The large brain size of the carnivore compared to rodents
necessitates a different approach to trimming and embedding thebrain; a recommended trimming scheme is presented in an STP
position paper by Bolon et al109 resulting in 7 sections/blocks.
The nomenclature of lesions of the nervous system in the dog
differs from the rodent most strikingly in the classification ofneoplasms, which is outside the scope of this document, given
the very low incidence of nervous system neoplasms in the young
age and well-controlled breeding programs of purpose-bred dogs
commonly used in general toxicity studies. For a detailed description of the neoplastic and tumor-like lesions, refer to “Tumors ofDomestic Animals”110 and other pertinent literature.
Table 10.1 summarizes the recommended terminology of
microscopic lesions observed in dogs used in toxicity studies.

Extramedullary Hematopoiesis—Choroid
plexus (Figures 10.1 and 10.2)
Other Term(s)Hematopoietic hyperplasia; myeloid hyperplasia; erythroid
hyperplasia; megakaryocytic hyperplasia; hematopoietic cellproliferation; myeloid metaplasia.

Pathogenesis/Cell of Origin
Hematopoietic progenitor cells.

Diagnostic Features
Hematopoietic cells of 3 lineages (myeloid, erythroid andmegakaryocytic) may be present.
Proportions of cell types varies.
Any of the lineages may be predominant.
Each lineage is often represented by cells in various
stages of differentiation.

Woicke et al.

49

Table 10.1. Microscopic Findings in the Nervous System: Brain, Spinal Cord, and Peripheral Nerve; Dog.
Brain, spinal cord, and peripheral nerve
Nonneoplastic
Neuron cell body
Decreased cellularity, neuron
Chromatolysis
Heterotopia, neuronal
Necrosis, neuron
Neuronophagia
Pigment, lipofuscin
Vacuolation, neuron
Neuron axon
Atrophy, axonal
Degeneration, axonal
Degeneration, nerve fiber
Dystrophy, axonal
Radiculoneuropathy
Glia cell bodyType II astrocytes
Astrocyte swelling/vacuolation
Astrocyte swelling
Astrocytosis
Gliosis (NOS)
Microgliosis
Pigment, lipofuscin
Satellitosis
Glia myelin
Accumulation, laminar, Schwann cell
Demyelination
Intramyelinic edema
Choroid plexus
Extramedullary hematopoiesisa
Vacuolationb
Vascularc
Necrosis/inflammation, media or wall, artery
Infarct
Thrombusb
GeneralCholesterol clefts
Cyst, squamous
Dilatation, ventriclea
Hemorrhageb
Hydrocephalus
Hydromyelia
Inflammationb
Infiltrate, [insert appropriate cell type]b
Mineralizationb
Syringomyelia
Syringomyelia/hydromyelia
Ectopic tissue
MeninxInflammationb
Mineralizationb
Peripheral nervous system
Accumulation, laminar, Schwann cell
Accumulation, matrix
Autophagy, neuron, ganglion
Necrosis/inflammation, media or wall, artery
Atrophy, axonal
Decreased cellularity, neuronalCommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
(continued)

50

Toxicologic Pathology 49(1)Table 10.1. (continued)
Brain, spinal cord, and peripheral nerve
Chromatolysis
Degeneration, axonal
Degeneration, nerve fiber
Demyelination
Dystrophy, axonal
Edema, intramyelinicb
Heterotopia, neuronal
Inflammationb
Infiltrate [insert appropriate cell type]b
Necrosis, neuronal
Neuronophagia
Vacuolation, neuronalb
Proliferative (nonneoplastic)
Meninx
Aggregates, granular cell
Meningioangiomatosisa
Peripheral nerve
Renaut bodiesa
Other cell lineages
Hamartoma, lipomatousabcCommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x

x
x
x
x

Terminology with diagnostic criteria and/or comments described in the text.
Terminology addressed in the General Pathology section.
Refer to description in the section “Cardiovascular System.”

Differential Diagnoses
Neoplasms of hematopoietic origin:
Leukemia, myeloid; Leukemia, erythroid; or Leukemia,
megakaryocytic:






Tumor cells are often all at one stage of differentiation
especially in less well-differentiated tumors.
Other organs are often involved with the same neoplastic cells.
Some leukemia present with minimal tissue involvement but with a high white blood cell count and numerous immature forms/blasts in the blood.
Mitosis, apoptosis, and necrosis may be present.

in almost 35% of 19- to 56-week-old males and 20% of 24- to
85-week-old females. 111 In dogs from certain breeders,increased incidences were noted, and the observation was notnecessarily associated with a pathologic condition. In anotherpublication, EMH was described in dogs euthanatized because
of seizures; no other remarkable hematopoietic findings werenoted and no other site of extramedullary hematopoiesis wasobserved.112 However, no coherent evidence for a connection
between EMH and seizures was provided.

Dilatation, Ventricle—Brain (Figure 10.3)
Other Term(s)Lymphoma:

Ventriculomegaly.



Pathogenesis/Cell of Origin

Cell morphology is different from that of myeloid, erythroid or megakaryocytic cells.

Unknown.
Infiltrate, mononuclear cell.
Mononuclear cells without any tissue reaction.

Inflammation:
Mononuclear cells with reaction of surrounding tissue.

Diagnostic Features

Widening of the ventricular spaces (bilateral or
unilateral).
Without changes in ependymal cells, white matter, glial/
microglial cells.
Normal cerebrospinal fluid flow.

Comment

Differential Diagnoses

Extramedullary hematopoiesis (EMH) in the choroid plexus
represents a very subtle finding in beagles; it has been noted

Hydrocephalus: Internal hydrocephalus is also characterized bya widening of the ventricular space; it is caused by disturbanceWoicke et al.
or blockage of ventricular drainage and accompanied by degenerative changes, including atrophy and cavitation of the whitematter.

Comment
Dilatation of the ventricle(s) or ventricular system is usually adiagnosis readily made at trimming. The need of histomorphologic correlation may require the repetition of the term. Vulloet al113 reported 17 asymptomatic beagle dogs with ventricular
dilation by magnetic resonance imaging and ultrastructure evaluations (normal ependymal cells, white matter, and glia/
microglial cells with normal cerebrospinal fluid flow).

51
Affected dogs exhibit ataxia, circling, rotatory nystagmus, mild
tetraparesis, proprioceptive deficits, or muscle atrophy.115,116

Renaut Bodies—Peripheral Nerve(Figure 10.5)
Biological Behavior
Benign proliferation of endoneural collagen forming welldemarcated elliptical, lamellar structures containing a core ofintensely eosinophilic fibers.

Other Term(s)Not applicable.

Meningioangiomatosis—Meninges(Figure 10.4)
Biological Behavior
Meningioangiomatosis is a benign proliferative condition of
the central nervous system, which is considered to be a vascularmalformation or hamartoma (ie, a disorganized, benign masscomposed of cells indigenous to the site) and not a neoplasm.

Other Term(s)Histogenesis
Mesenchymal origin endoneural collagen subtypes I, II and VI)
and mucopolysaccharides

Pathogenesis
Physiologic adaptation of nerves (usually distal portion of
nerves within fore and hind legs) hypothesized to be a response
to mechanical stress imposed on them.

Not applicable.

Diagnostic Features
Pathogenesis

This finding likely develops from mesenchymal, pluripotent
cell with fibroblastic or meningothelial differentiation.

Diagnostic Features








Plaque-like thickening of the meninges of brain stem or
cervical spinal cord of young dogs, which can be discerned grossly.
Characterized by a focal proliferation of blood vessels
and spindle-shaped meningothelial cells extending fromthe meninges along Virchow-Robin’s spaces into theadjacent brain parenchyma.
Cells do not display atypia, pleomorphism, or mitotic
activity.
Meningothelial cells especially those adjacent to theblood vessels are immunoreactive for vimentin.
Dense gliosis, rarefied neuropil, scattered Rosenthal
fibers (thick, elongated, brightly eosinophilic, irregularstructures in astrocyte processes), and slight white matter vacuolation are the accompanying features.

Comment
Meningioangiomatosis is a rare benign proliferative condition ofcentral nervous system reported in humans and animals including young dogs. It is generally considered to be a hamartoma;
however, whether meningioangiomatosis is hamartomatous,
neoplastic, or a developmental defect remains controversial.114



Subperineurial accumulations of elongate, loosely
whorled, mucin-rich, cell-poor connective tissue that
occur in various peripheral nerves, especially in the distal branches of the sciatic nerve branches tibial and fibular nerve, and less commonly in autonomic nerves nearviscera.117
Renaut bodies (RBs) occur within nerve fibers but do
not cause axonal degeneration or reactive gliosis/inflammation or encapsulation.
They range from 20 to 500 mm in diameter and hundredsof microns in length in longitudinal section
They are elliptical in shape when sectioned transversallyand cylindrical in shape longitudinal and have onionskin arrangement containing loosely textured, concentric lamellae of filamentous strands intermixed withsparse numbers of dark spindle shaped nuclei.

Special diagnostic techniques:Histochemical results included positive acidic Alcian
blue, Gomori’s trichrome, and Verhoeff Van Gieson’sand negative Periodic acid–Schiff, Congo red, andLuxol fast blue/cresyl violet.
Immunohistochemical results included positive vimentin and collagen (subtypes I, II, and VI) and negative
neuron-specific enolase, S-100, glial fibrillary acidic
protein, amyloid A component, desmin, alphasarcomeric actin, pancytokeratin, epithelial membrane
antigen, and von Willebrand factor.

52

Toxicologic Pathology 49(1)Transmission electron microscopy revealed loosely
arrayed, circumferentially oriented collagen fibers intermixed with varying amounts of amorphous substanceand finely fibrillar material. Most of the cells comprising
the RBs are fibroblasts.

Differential Diagnoses
Schwannoma, mesenchymal neoplasia.

Comment
Renaut bodies have been described in humans at sites ofchronic nerve entrapment 118 as well as in control beagle
dogs117,119,120 and horses.119,121 The incidence of RB can
increase in subchronic and chronic studies120 and have beenreported in hypertrophic neuropathy.122 These lesions may
serve to cushion adjacent nerve fibers from mechanicaltrauma.119 This premise is supported by the experimental
demonstration that RB may be induced by nerve compression,120,123,124 suggesting that mechanical trauma is the primary pathogenesis for the change, although other mechanisms
may be involved since affected autonomic nerves near viscera
are not subjected to great mechanical stress.117 The number of
RB increases with age125 and may vary by the vendor.126

11. Female Reproductive Tract—Ovaries,
Oviduct, Uterus, Cervix, and Vagina
Introduction
For general aspects on the female reproductive system, refer to therodent publication. This chapter provides the recommended nomenclature for the canine female reproductive tract. Generally usedpreferred terms of systemic nonproliferative lesions across organsystems, such as hemorrhage, inflammation, or thrombus, areincluded in separate INHAND manuscripts and are therefore notincluded in this section, except where there are distinguishing features of the process relevant to the female reproductive system. In allorgans and tissues, including those of the female reproductive system, tumors are very rarely observed in toxicologic pathology studies in beagle dogs and therefore are generally not included in thismanuscript. For the nomenclature and diagnostic criteria of neoplastic changes, refer to the World Health Organization’s Histologic
Classification Tumors of the Genital System of Domestic Animals,127 the more recent publication on Tumors of Domestic Animals by Meuten1, and pertinent literature.

Briefly, the cycle is composed of proestrus (lasting 1-2
weeks), estrus (lasting 1-2 weeks), and diestrus (lasting
between 2 and 3 months). Metestrus in the dog is a very shortphase usually not distinguished from estrus.102 The anestrus
phase of the cycle is unique to the dog. It represents a quiescentphase following diestrus and lasting between 3 and 5 months.102
Dog-specific peculiarities in the reproductive physiology
include the presence of polyovular follicles, ovulation of animmature oocyte, and a periovulatory period with particularlyhigh concentrations of circulating progesterone.129–131
In the female dog, the ovary, uterus, cervix, and vagina
show marked cyclical changes, which are displayed inTable 11.1. For easy reference, the cycle changes in the mammary gland are included as well.

Nomenclature
The recommended nomenclature for nonproliferative and nonneoplastic proliferative changes in beagle dogs used in general
toxicology studies listed in Tables 11.2 to 11.6. The nomenclature is adapted from the rodent manuscript as listed on the
go-RENI.org website. Diagnostic criteria for nonneoplasticchanges are only described if they differ from those used inmice and rats.

Ovaries
Adenoma—Ovary
Comment
A rare benign tumor in the ovary of dogs, which has been
observed in toxicologic pathology studies in beagle dogs. In
the INHAND rodent manuscript of the female reproductive
system, tubulostromal adenoma and cystadenoma of theovary are included.132 For the diagnostic criteria in dogsof this and other neoplastic findings, refer to the fascicleon the genital system of the International Histological Classification of Tumors of Domestic Animals published by the
Armed Forces Institute of Pathology in conjunction with theAmerican Registry of Pathology and the World HealthOrganization,127 the chapter on neoplasms of the genital
systems in the textbook Tumors of Domestic Animals133,
and/or current literature.

Oviduct, Uterus, Cervix, and Vagina
Hyperplasia, Segmental Cystic—Uterus (Figures 11.1–11.4)
Other term(s)Anatomy and Physiology
The anatomy and histology of the canine female reproductive
tract has been described extensively, and it is beyond the scopeof this chapter to detail the morphology of this system and its
cycle-dependent changes. The reviews by Rehm et al102 andChandra and Adler128 are excellent references for this topic.

Pseudoplacentational endometrial hyperplasia (PEH).

Pathogenesis/cell of origin
Endometrial glands and stroma under the influence ofprogestogen stimulation.

Woicke et al.

53

Table 11.1. Normal Cyclic Changes in the Female Reproductive System.
Stage of thecycle

Duration

Ovary

UterusVaginaMammary gland

Immature

Puberty around 6
months of age

Atrophic

2-3 cells thickNo glandular
structures

Proestrus

7-9 days

No corpus luteum
Waves of follicular
growthMedium-large
follicles
Large follicles/startof luteinization

Mild edema and congestion

Thickened

Large mature
corpus luteum
Early: brighteosinophilic
Later: vacuolated
Degenerate corpus
luteumApoptosis, pigment
deposit

Variable thickness, þ/
Two zones
leukocytes (neutrophils)(i) Inner compact (vacuolated
cells, folding of surface)(ii) Outer spongy (dilatation ofglands)Atrophic
2-3 cells thick

Inactive glands/ductdilatation
Stromalproliferation
with edema
Secretion þþþ

Estrus/metestrus 9-10 days

Diestrus

60-90 days

Anestrus

4.5-5 months

Edema and gland development Keratinization andsubmucosal edema

Diagnostic features








Localized proliferation of the endometrium
Broad-based polypoid mass extending into the lumen or
continuous band of protruding tissue.
Cystic epithelial proliferation of the endometrium separated into 2 layers.
The uterine surface endometrium is replaced by cavernous spaces lined by hyperplastic columnar epithelium
with pale pink secretory cytoplasm.
The lower endometrium contains dilated basal glands.
Necrotic luminal surface and necrotic debris often
observed in the lumen.

Differential diagnosesEndometrial stroma polyp. The polyp contains substantial connective tissue stroma in addition to dilated glands
and is pedunculated.

Comment
Segmental cystic endometrial hyperplasia or PEH of pseudopregnancy occurs occasionally; it has been observed inassociation with pyometra. It may be induced by a variety
of sterile substances in the uterine lumen during theluteal phase134,135 and intrauterine Escherichia coli suspension.136 The ovary contains prominent and persistentcorpora lutea. Use of the term deciduoma, while sometimes present in the literature,137 is not recommended as it
is not descriptive and does not reflect the pathogenesisof the finding. 1 3 8 Likewise, the use of the term

Inactive remnants

pseudopregnancy is not recommended and should be
restricted to the clinic.

Polyp, Endometrial Stroma—Uterus
Other term(s)Endometrial polyp.

Pathogenesis/cell of origin
Proliferation of glandular and stromal elements.

Diagnostic featuresSingle or multiple projections of sessile or pedunculatedmasses into the uterine lumen.
Dilated uterine glands within a proliferating stroma.
Pedunculated polyps may protrude into the cervix andvagina.

Differential diagnoses

Segmental cystic hyperplasia. The polyp contains substantial connective tissue stroma in addition to dilated
glands and is pedunculated.

Comment
In the rodent (see the rodent publication or the goReni website),endometrial stroma polyps are considered benign neoplasms.
In the dog, as in other domestic animals, they can be observed
as a spontaneous change that is considered hyperplastic innature and not a neoplastic entity.

54

Toxicologic Pathology 49(1)Table 11.2. Microscopic Findings in the Female Reproductive System: Ovary, Dog.
Ovary
Congenital
Ectopic tissuea
Ovotestis
Nonneoplastic
AmyloidAngiectasis
Atrophy
Atrophy, age-related
Atrophy, corpora lutea
Cyst, bursal
Cyst, epithelial
Cyst, follicular
Cyst, luteal
Cyst, NOS
Cyst, paraovarian
Cyst, rete ovarii
Decreased number/absent corpora lutea
Decreased number/absent follicles
Degeneration, corpora lutea
Degeneration, oocyte
EdemaaFollicle, luteinizedb
Follicle, polyovular
Hypertrophy, corpora lutea
Hypertrophy, interstitial cell
Immature
Increased number, atretic follicles
Increased number, corpora lutea
Infiltrate [insert appropriate cell type]
Inflammation, ovary
Mineralizationa
Pigmenta
Vacuolation, corpora luteaa
Vacuolation, granulosa cella
Vacuolation, interstitial cella
Vacuolation, theca cella
Proliferative (nonneoplastic)
Hyperplasia, cystic/papillary
Hyperplasia, granulosa cell
Hyperplasia, interstitial cell
Hyperplasia, rete ovarii
Hyperplasia, Sertoli cell
Hyperplasia, sex cord stromal, mixed
Hyperplasia, smooth muscle, mesovarium
Hyperplasia, theca cell
Hyperplasia, tubulostromal
Neoplastic
Adenomac

CommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

Terminology addressed in the General Pathology section.
Preovulatory follicles are luteinized in the dog and should not be diagnosed as a finding if the dog is in late estrus.
cTerminology with diagnostic criteria and/or comments described in the text.
b

12. Male Reproductive System—Testis,
Efferent Ducts, Epididymis, and Prostate
Introduction
Standardized nomenclature recommended for the reporting of
microscopic lesions in the canine male reproductive tract isbased on the INHAND project nomenclature previously published for the rat and mouse male reproductive system.139
The male dog reproductive anatomy and histologic architecture differ from the rodent in several ways. Rodents canvoluntarily retract the testes into the inguinal canal; dogs cannot. Canine testes that have not descended from the inguinalWoicke et al.

55

Table 11.3. Microscopic Findings in the Female Reproductive System: Oviduct, Dog.
Oviduct

CommonUncommonNonneoplastic
Atrophy
Infiltrate [insert appropriate cell type]aInflammationa
Salpingitis isthmica nodosa
Proliferative (nonneoplastic)
Hyperplasia, epitheliuma

Not observed but potentially relevant

Not applicable

x
x
x
x
x

Terminology addressed in the General Pathology section.

Table 11.4. Microscopic Findings in the Female Reproductive System: Uterus, Dog.
UterusCongenital
Mesonephric duct remnants
Nonneoplastic
Abscess(es)AmyloidAngiectasis
Aplasia, segmental
Apoptosisa
Atrophy
Cyst, NOS
Decidual reaction
Decidualization, focal
Dilatation, glandular, cystic
Dilatation, luminal
Fibrosis
Granuloma
Hypertrophy, epithelium
Hypertrophy, myometrial
Hypoplasia
Infiltrate [insert appropriate cell type]aInflammationa
Inflammation, endometrium
Inflammation, myometrium
Metaplasia, squamous cell
Necrosisa
Pigmenta
Prolapse
PyometraVacuolation, epitheliuma
Proliferative (nonneoplastic)
Adenomyosis
Hyperplasia, angiomatous
Hyperplasia, endometrial stromal
Hyperplasia, endometrial, diffuse
Hyperplasia, glandular, cystic
Hyperplasia, glandular, focal
Hyperplasia, granular cell
Hyperplasia, myometrial
Hyperplasia, segmental cysticb
Polyp, endometrial stromalbabCommonUncommonNot observed but potentially relevant

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

Terminology addressed in the General Pathology section.
Terminology with diagnostic criteria and/or comments described in the text.

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

Not applicable

56

Toxicologic Pathology 49(1)Table 11.5. Microscopic Findings in the Female Reproductive System: Cervix, Dog.
CervixCommonCongenital
Remnants, mesonephric duct
Nonneoplastic
Aggregate, granular cells
Atrophy, epithelium
Cyst, NOS
Degeneration, epithelium
Erosion/ulcer
Infiltrate [insert appropriate cell type]aInflammationa
Keratinization, increased
Mucification, increased
Necrosis, epitheliuma
Prolapse
Vacuolation, epitheliuma
Proliferative (nonneoplastic)
Adenomyosisb
AdenosisHyperplasia, epithelium
Hyperplasia, granular cell
Hyperplasia, stroma
Hypertrophy stromaab

UncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

Terminology addressed in the General Pathology section.
For a description and diagnostic criteria, refer to Uterus (rodent).

Table 11.6. Microscopic Findings in the Female Reproductive System: Vagina, Dog.
VaginaCommonCongenital
Imperforate vagina
Prostatic rudiment
Remnant, mesonephric duct
Nonneoplastic
Aggregate, granular cell
Atrophy, epithelium
Cyst, NOS
Degeneration, epithelium
Erosion/ulcer
Infiltrate [insert appropriate cell type]aInflammationa
Keratinization increased
Mucification increased
Necrosis, epithelium
Prolapse
Vacuolation, epitheliuma
Proliferative (nonneoplastic)
AdenosisHyperplasia, epithelium
Hyperplasia, granular cell
Hyperplasia, stroma
Hypertrophy stromaaUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
xa
x
x
x
x
x

Terminology addressed in the General Pathology section.

canal or abdominal cavity by about 6 weeks of age indicate
cryptorchidism.140 Unlike the funnel-like pattern in rodents
where efferent ducts join together as a single common duct

emptying into the head of the epididymis, dogs have multiple,
short parallel efferent ducts emptying separately into thehead.141 Congenital blind-ending efferent ducts in the dog mayWoicke et al.
predispose to sperm granulomas (via sperm stasis) near the
epididymal head.142 The prostate is the only accessory sex
gland in the male dog. Anatomy of the canine penis includes
an os penis (located dorsal to the urethra) and bulbus glandis(erectile tissue) which during mating contribute to copulationand lock (“tie”) the male and female, respectively.

Sexual Maturation in the Male Dog
The use of animals with sexually immature or peripubertal
reproductive organs is not a sensitive means to detect potential
treatment-related effects to adult reproductive tissues. Dogs may
be 5 to 6 months of age at the start of routine toxicity studies, but
sexual maturity occurs from 7 to 12 months of age, and this age
can vary depending on the source of the animals.140,143 Therefore, in male dogs at the lower end of this age spectrum, the male
reproductive tract may often be undergoing sexual maturation,
particularly during nonclinical studies of short duration (lessthan 13 weeks). Therefore, the reproductive tract of dogs may
not be mature in some nonclinical studies. Unlike rats where
spermatogenesis begins shortly after birth, in dogs, there is apostnatal dormant period of about 5 months before spermatogenesis starts.140 At 5 to 6 months of age, the canine Sertoli cells
begin to produce fluid which opens tubular lumina and may form
vacuoles within the seminiferous epithelium.140 Spermatocytes
are evident at 6 to 7 months of age, followed by spermatids at 6
to 8 months of age, and then shedding spermatozoa at 7 to 10
months of age.140 Early waves of spermatogenesis are not as
efficient so there can be frequent germ cell sloughing, degeneration, and multinucleated cells with cellular debris present within
the epididymis.144 This is often not uniform across the canine
testes since the individual seminiferous tubules vary in their rateof spermatogenesis.144 This is in contrast to rodents where therate of spermatogenesis is fairly uniform across the testes.140
In contrast to rodents, dogs on a nonclinical study do notmature at the same rate.140 Despite a similar age range, somedogs on study may have mature testes and epididymal ducts
filled with sperm, whereas other dogs may have immature
reproductive tracts or peripubertal seminiferous tubules with
germ cell depletion and/or degeneration and epididymides
devoid of spermatozoa.140 Therefore, dogs on study may havevarying microscopic presentation attributable to differences intheir individual rate of sexual maturation and not due to test
article administration. It is important to distinguish microscopic differences due to immaturity or peripuberty from thoserelated to experimental treatment. The pathology report should
address when it is challenging or not possible to differentiate
germ cell degeneration attributed to sexual maturation fromthat which could be a treatment-related effect.
Documentation of sexual maturation can aid the interpretation of study findings. A survey by a Working Group of the
Scientific and Regulatory Policy Committee of the STP
showed that pathologists prefer the terms “immature” and“mature” and also use the term “peripubertal.”145 However,some institutions may only record sexual immaturity since thedefault is reproductive maturity. The Canine Nonrodent

57
Working Group of the INHAND project recommends that for
nonclinical studies the state of sexual maturity should berecorded as “mature” when sexually mature and “immature”
or “peripubertal” when sexually maturity has not been reached.
These terms more accurately reflect the nature of changes
related to an underdeveloped male reproductive tract and will
help to better distinguish findings related to experimental treatment from those related to sexual maturation. Evaluation of theperipubertal testis is challenging, if not impossible, to assessfrom treatment-related findings to spermatogenesis. Whenrecording maturation status, the pathologist should clearly referto sexual maturation to avoid confusion with skeletal maturation. Where in the study data the state of sexual maturation isrecorded which should be at the discretion of the institution.
The INHAND project recommends recording sexual maturity
as “immature,” “peripubertal,” “mature,” or, when sexual
maturity cannot be determined, “indeterminate” under a“summary tissue,” as proposed by a working group of the STPor under the testis if a summary tissue is not available.
When assessing the maturation status, it is important to beaware that the canine male reproductive tract is not synchronized with respect to the rate of maturation of the testes, epididymides, and prostate. Maturation of the prostate may bedelayed in relationship to the state of sexual maturation in thetestes or vice versa. The difference in the state of maturation
between the testes and prostate has been attributed to differences in the expression of the androgen receptor.140 Pathologists usually identify testicular maturation by spermatogenesis(ie, presence of elongating spermatids) and epididymal tailcontaining spermatozoa.144,145 Immature prostatic acini havenarrow, undeveloped lumina lined by basophilic epitheliumthat is cuboidal to flattened with a high nuclear to cytoplasmic
ratio.49,143 The amount of connective tissue is relatively higher
in the immature prostate than the mature prostate.143 Maturing
prostate can have a patchy distribution of immature small aciniwith wide interstitium and mature glands with scant stroma.
Mature prostatic glands are dilated with epithelial infoldings
lined by columnar cells with abundant eosinophilic cytoplasm
and supported by scant stroma.143 Luminal eosinophilic secretory material is often not observed.
Brief definitions of the three separate stages of sexual
maturation for the male dog are:
Immature: Immature animals have undeveloped testes, epididymides, and prostate. The immature testes lack spermatozoa within seminiferous tubules. From birth to about 5
months of age, the immature seminiferous tubules are contracted and populated only by Sertoli cells and centrally
located gonocytes.144,146 Subsequently, the underdeveloped seminiferous tubules are lined only by Sertoli cells
and basally located spermatogonia.144 There can be low
numbers of spermatocytes. Spermatids and spermatozoa
are not present. There are no or rare exfoliated germ celldebris and multinucleated cells. Immature epididymides
have underdeveloped contracted epididymal ducts lined
by a low epithelium.49,144,146 Mature sperm are not present,

58

Toxicologic Pathology 49(1)but there may be rare germ cell debris in the lumina. The
immature prostate is small and glands are not developed.
Peripubertal: Peripubertal animals have seminiferous
tubules in varying stages of development or may havedeveloped testes but underdeveloped prostate. Seminiferous tubular lumina are partially dilated and there can bevacuoles within the seminiferous epithelium due to fluid
production by Sertoli cells.140,144 Tubular epithelium
may vary from having a limited number of germ cells
to being fully populated with germ cells. Some seminiferous tubules have germ cell development to mature
spermatids. There is frequent degeneration of germ cells
or multinucleated giant cells during early waves of spermatogenesis.144 At or near the rete testis, there may be
sperm stasis. Contracted or partially expanded epididymal ducts may contain increased numbers of sloughed
degenerate germ cells evident as luminal cell debris.144
Few spermatozoa may be present in the epididymal heador body but tend to be particularly absent from the epididymal tail. Maturation of the prostate may vary fromthe maturation state of the testes since prostatic maturation is not synchronized with testicular development.144
If the testes are mature but the prostate still underdeveloped, the dog should be considered as peripubertal.
Mature: Mature animals have developed testes and prostate.
Mature testes have a full complement of developed seminiferous tubules with all 4 generations of germ cells (spermatogonia, spermatocytes, round spermatids, andelongating spermatids) present and mature spermatids
present throughout the section. Residual bodies are present
and signify recent release of mature spermatids.140 Germcell degeneration and multinucleated giant cells are lessthan observed for peripubertal testes.144 Abundant spermatozoa generally expand the lumina of epididymal ductsin the corpus and tail.144 There can be rare sloughed germ
cells. Maturation of the prostate may vary, but in sexuallymature animals, the prostate should be fully developed. In
contrast, animals with developed testes but underdeveloped prostate are considered peripubertal.

as it increases the opportunity to include the efferent ducts forevaluation, since they are located within the epididymal head ofthe dog.140 Pathologists should be aware that the efferent ducts
are embedded within the initial segment/head of the epididymis
in dogs and not erroneously diagnose these normal structures as
epididymal degeneration or epithelial hyperplasia. Additional
reproductive tissues may be examined for specific studies but
are not generally necessary to adequately assess the male reproductive tract in routine toxicity studies.

TestisCanine seminiferous tubules are grouped into lobules separated
by connective tissue and with ends emptying into a central retetestis within the fibrous mediastinum.140 This histologic architecture influences how testicular injury morphologically presents in the dog. For example, injury to an affected
seminiferous tubule can present as a change isolated to a lobule.
In contrast, rats do not have lobular arrangement so morphologicchanges to a seminiferous tubule may be dispersed throughout
the testis.140 Seminiferous tubule and/or stage-specific microscopic findings should include modifiers such as cell type (eg,Sertoli cells, germ cells, spermatogonia, spermatocytes, roundspermatids, or elongating spermatids). Microscopic evaluationof the canine testis should be conducted with an attention tospermatogenic stages.147,148 However, detailed stage aware evaluations are challenging for the dog140 due to the following
reasons: (1) germ cell layer associations are less synchronizedthan in the rat, (2) spermiation (release of mature spermatids)
may occur over 2 stages, (3) the profile of elongating spermatid
heads may present differently depending on the plane of section,(4) periodic acid—Schiff does not stain the spermatid acrosome
until spermiation,149 and (5) dogs have a higher incidence of
spontaneous findings (eg, hypospermatogenesis, swollen spermatocytes, multinucleated giant cells, or changes associatedwith peripubertal development) compared to the rat.
The recommended terminology of microscopic changes
observed in the testes of the dog is presented in Table 12.1.

Hypoplasia, Tubule—Testis (Figures 12.1 and
12.2)
Morphologic Evaluation of Canine Male Reproductive
SystemExternal genitalia (penis, prepuce, and scrotum), the spermatic
cord (vas deferens, pampiniform plexus, cremaster muscle), andefferent ducts are generally not examined microscopically inroutine nonclinical toxicity studies. The male reproductive tissues routinely evaluated in nonclinical toxicity studies are thetestes, epididymides, and prostate. Histologic sectioning of thetestis should include the tunica albuginea, parenchyma (seminiferous tubules and interstitium), and the rete testis. Microscopic
examination of the epididymides should include the head(caput), body (corpus), and tail (cauda). Microscopic findingsin the epididymis should include location modifiers such as heador tail. Longitudinal sampling of the epididymides is preferred,

Other Term(s)Atrophy/hypoplasia, focal hypoplasia, Focal subcapsular tubular atrophy or hypoplasia, segmental hypoplasia, Sertoli cellonly tubules.

Modifiers
Segmental.

Pathogenesis
The lobular distribution suggests that the group of tubular profiles belong to a single, coiled, seminiferous tubules never
having been populated by germ cells, hence hypoplasia.

Woicke et al.

59

Table 12.1. Microscopic Findings of the Male Reproductive System: Testis; Dog.
TestisCommonCongenital
Aplasiaa
Cryptorchidism
Hypoplasiaa
Hypoplasia, tubuleb
Nonneoplastic
Amyloida
Angiectasis
Atrophy, Leydig cell
Atrophy, tubule
Degeneration, germ cell
Degeneration, tubulec
Degeneration/atrophy, tubule
Depletion, germ cellc
Dilatation, rete testis
Dilatation, tubule
EdemaaExfoliation, germ cell
Fibrosisa
Hypospermatogenesisb
Infiltrate [insert appropriate cell type]aInflammationa
Mineralizationa
Multinucleated giant cells
Necrosis, Leydig cell
Necrosis, testis
Necrosis, tubule
Necrosis/inflammation, vascular/perivasculard
Pigmenta
Residual bodies, atypical
Sperm granuloma
Sperm stasis
Spermatid retention
Spermatocele
Swollen spermatocytesb
Vacuolation, Leydig cell
Vacuolation, macrophage
Vacuolation, tubule
Proliferative (nonneoplastic)
Hyperplasia, Leydig cell
Hyperplasia, mesothelium
Hyperplasia, rete testis

UncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

Terminology addressed in Systemic Pathology section.
Terminology with diagnostic criteria or comments described in the text.
c
Finding more frequent as an induced change.
dTerminology addressed in the Cardiovascular section.
b

Diagnostic Features






Lobular distribution that is often wedge-shaped and situated in a subcapsular location.
Generally affects triangular clusters (segments) of varying numbers of seminiferous tubular profiles.
Affected tubules are completely devoid of germ cells
and are only lined by Sertoli cells.
Tubule diameter smaller than normal tubules.
Tubular lumina are empty.



Affected tubules reside immediately adjacent to normal
tubules.
Unilateral, but may be bilateral.

Differential Diagnoses


Atrophy, tubule: Alternatively, seminiferous tubulesthat have sustained permanent loss of germ cells (atrophy) may result in Sertoli cell-only tubules. Basement
membrane thickening may be a feature.

60

Toxicologic Pathology 49(1)Transitional areas (tubuli recti): Germ cells are reducedor absent at transitional areas between seminiferous
tubules and the rete testis.150 These areas are normal
anatomy and not a finding to be diagnosed.
Hypospermatogenesis: A complete or partial loss of one
or more germ cell layers that tend to have a patchy
distribution throughout the testes.



Differential DiagnosesComments
Tubules lined by only Sertoli cells in dogs are attributed tohypoplasia of entire tubules or tubular segments presumably
never populated by germ cells during testicular development.
The lobular distribution is considered evidence that theaffected tubular profiles belong to a single, convoluted, seminiferous tubule.144 Descriptive terminology such as “Sertoli
cell-only tubules” have been used or suggested by someauthors,49,151 and the finding can be confused with focal tubular atrophy. However, the high prevalence of this finding indogs less than 1 year of age suggests “tubule hypoplasia” is anappropriate term. Tubule hypoplasia may be seen in dogs ofany age. The etiology is presumably due to a reduced migrationof germ cells to the affected segment.









Hypospermatogenesis—Testis (Figures 12.3
and 12.4)

Other Term(s)Segmental hypospermatogenesis.
Modifiers
Segmental.

Pathogenesis
Hypospermatogenesis is attributed to transient failure/inefficiency of spermatogenesis affecting a segment of the seminiferous tubule resulting in an absence of one more generation ofgerm cells.

Affected tubules may have prominent Sertoli cells orcontain small amounts of luminal debris, multinucleatedgiant cells, and/or swollen spermatocytes.
Unilateral or bilateral.

Immature: The presence of elongate spermatids inhypospermatogenesis differentiates this finding fromimmaturity.
Peripubertal: Seminiferous tubules vary from having alimited number of germ cells to being fully populatedwith germ cells including development to mature spermatids. There is frequent degeneration of germ cells or
multinucleated giant cells.
Transitional areas (tubuli recti): Germ cells are reducedor absent at transitional areas between seminiferous
tubules and the rete testis.150 These areas are normal
anatomy and not a finding to be recorded in a nonclinical
toxicology study
Hypoplasia, tubule: Triangular clusters (segments) ofvarying numbers of seminiferous tubular profiles completely devoid of germ cells and are lined by Sertoli cells
only.
Atrophy, tubule: Permanent loss of germ cells (atrophy)
may result in Sertoli cell-only tubules. Basement membrane thickening may be a feature.
Degeneration/atrophy, tubule: Some seminiferous
tubules have degenerating germ cells and other tubules
have partial or complete loss of germ cells that may
result in Sertoli cell-only lined tubules.
Degeneration, tubule: Seminiferous tubules have degenerating germ cells not restricted to a specific germ cell
type or stage.
Degeneration, germ cell: Cell type-specific or stagerestricted germ cell death demonstrating cytoplasmic
eosinophilia, chromatin condensation/margination,
and/or nuclear fragmentation.139
Depletion, germ cell: Partial or complete loss of one ormore germ cells layers in a more specific pattern and/or
diffuse pattern.139 Depletion related to treatment istypically a diffuse change to a uniform population ofgerm cells.

Comments
Diagnostic Features







Affects individual tubular profiles (segments) randomly
distributed throughout the testes (may affect few, many,or the majority of tubules).
Tubules have a complete or partial absence of spermatocytes, round spermatids, and/or elongate spermatids.
When affecting only some of the germ cell generations,the affected tubular segments may be missing roundspermatids and/or pachytene spermatocytes from thelayers below the overlying elongate spermatids.
There may be reduction of tubular diameter.

It is common for most testes to have a few tubules affected bysegmental hypospermatogenesis. The finding varies widely inseverity, with the lesion being marked or severe in some animals.144,150,151 It has been reported to range from affecting<10% tubules per testis to greater than 75% of tubules per
testis.144 At higher severity levels, the size and/or weight of
the testis may be reduced and the number of sperm in theepididymides may be reduced.150 Distinguishing hypospermatogenesis from peripubertal testes or tubule degeneration/atrophy or germ cell depletion can be challenging. There is often
overlap in the appearance of germ cell depletion andWoicke et al.
hypospermatogenesis. Germ cell depletion that is induced byxenobiotics or radiation tends to have a more widespread, diffuse distribution and often impacts a more uniform populationof cells. In addition, the extent of the finding tends to correlatewith the length of the treatment period. In contrast, hypospermatogenesis has a more random distribution throughout thetestis, often affects different germ cell types, and the severity
and/or cell types affected may not correspond to the treatment
period. Hypospermatogenesis has been reported to occur at ahigher incidence in younger control beagle dogs151; however,
the absence of various germ cell types is a normal featureduring puberty, and when diagnosing hypospermatogenesis,
there should be clear evidence of maturity (eg, mature spermatids, residual bodies, expansion of the epididymal tail). Since
hypospermatogenesis is a transient inefficiency in spermatogenic activity, degenerate/multinucleated germ cells are infrequent.140 The appearance of an increased incidence and/or
severity of hypospermatogenesis with treatment may occur due
to test article-related germ cell depletion. Therefore, testicularevaluations must consider dose-related exacerbation of thischange and it is important to be aware of historic control data
for the test site.

Swollen spermatocytes—Testis (Figure 12.5
and 12.6)
Other Term(s)Enlarged spermatocytes.

Pathogenesis
Swollen spermatocytes are considered to be individual germ
cells arrested during meiosis. These are likely arrested secondary spermatocytes that failed to undergo their second meiotic
division. These swollen spermatocytes are often sequestered
within a normally developing germinal epithelium.

Diagnostic Features








Abundant pale cytoplasm with eccentrically locatednuclei.
Hyperchromatic nuclei (pyknotic) due to chromatincondensation.
Often mononuclear though authors have also shown
binucleate.
Affects spermatocytes as single cells or as loose aggregates within a portion of the seminiferous tubule and aresurrounded by germ cells undergoing normal maturation
to round spermatids.
Affects a few tubules per testis.
Often bilateral.

Differential DiagnosesMultinucleated giant cells: Multinucleated giant cells
composed of spermatogenic cells (often round61

spermatids) reside within the seminiferous epithelium
or lumen.139
Degeneration, germ cell: Cell type-specific or stagerestricted germ cell death demonstrating cytoplasmic
eosinophilia, chromatin condensation/margination, or
nuclear fragmentation.139

Comments
Swollen spermatocytes are a common incidental finding that
can have a high prevalence reported to affect up to 93% of
control beagle dogs that are 6 to 36 months of age.151 The
lesion is generally present at a low severity level affecting a
few tubules per testis. Swollen spermatocytes are postulated to
be secondary spermatocytes that did not undergo the second
meiotic division and survived to later stages.140 A low severityof swollen spermatocytes is generally not diagnosed. However,when the change is a prominent feature, diagnosis of swollen
spermatocytes may be warranted.

Excurrent Ducts (Efferent Ducts
and Epididymis)
In the dog, sperm are not typically observed in the head andproximal body of the epididymis and only become evident asthey become more concentrated in the body and tail.144 This isa normal feature in the dog and should not be recorded as afinding. One or more intranuclear eosinophilic inclusion bodies
may be present in any region of the epididymis, but the head isconsidered the most common location.152,153 Ultrastructurally,
these inclusions have been reported to resemble secondary
lysosomes and their significance is unknown.152,153
Table 12.2 summarizes the histopathological lesions thatcan be observed in the efferent ducts and epididymis of beagle
dogs in nonclinical toxicological studies.

Adenomyosis—Efferent Ducts
and Epididymis
Other Term(s)Diverticulum, epithelial.

Pathogenesis
Origin unknown.

Diagnostic FeaturesPeriductular connective tissue contains epithelial diverticuli of ducts that have penetrated through the smoothmuscle layer.
Entrapped sperm may be within the lumen of epithelial
diverticuli.
Sperm granuloma may occur secondary to entrappedsperm.

62

Toxicologic Pathology 49(1)Table 12.2. Microscopic Findings of the Male Reproductive System: Efferent Ducts and Epididymis; Dog.
Efferent ducts and epididymis
Congenital
Aplasia
Hypoplasiaa
Nonneoplastic
Adenosis
Amyloida
Atrophy duct
Cell debris, lumen
Cribriform change
Degeneration, epithelium
Dilatation, duct
EdemaaInfiltrate [insert appropriate cell type]aInflammationa
Karyomegaly
Metaplasia, squamous cell
Necrosis/inflammation, vascular/perivasculard
Single-cell necrosisa
Sperm granuloma
Sperm stasis
Sperm, decreased, lumenb
Spermatocele
Vacuolation, epithelium
Proliferative (nonneoplastic)
Adenomyosisc
Hyperplasia, epitheliumc

CommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

Terminology addressed in Systemic Pathology section.
Finding more frequent as an induced change.
cTerminology with diagnostic criteria or comments described in the text.
dTerminology addressed in the Cardiovascular section.
b

Differential Diagnoses

Pathogenesis

Not applicable.

Origin unknown.

Comments

Diagnostic Features

Adenomyosis in the efferent ductules, epididymis, or vas deferens (deferent duct) is due to diverticula of epithelium that has
extended through the smooth muscular wall supporting theducts.154,155 Adenomyosis should not be confused with intraepithelial lumina and is distinct from adenosis of rodents,
which are groups of epithelial cells with microlumina between
ducts. The epithelial diverticuli in dogs may contain entrappedspermatozoa and are thought to contribute to the formation of
sperm granuloma at locations considered unrelated to blindended efferent ducts near the epididymal head.154 Estrogen has
been suggested to play a role in adenomyosis.155

Proliferation of the epithelial cells lining the epididymis.

Hyperplasia, Epithelium—Efferent Ducts andEpididymis
Other Term(s)Not applicable.

Differential Diagnoses

Cribriform change: The epididymis more commonly
responds with cribriform change rather than epithelial
proliferation. Cribriform change is attributed to collapseof epididymal ducts140 associated with infoldings thatcan form 50-200 mm intraepithelial spaces (pseudoglandular cysts) lined by partially compressed epithelium. 151 Pseudoglandular spaces may containeosinophilic content. Cribriform change has been
described as a background finding in the dog at thejunction of the epididymal body and tail.144 Cribriform
infoldings may also occur at the epididymal head in
association with obstruction of the efferent ducts.140
Sometimes cribriform change is considered a hyperplastic lesion.140

Woicke et al.

63

Table 12.3. Microscopic Findings of the Male Reproductive System: Prostate; Dog.
Prostate
Congenital
Aplasia
Hypoplasiaa
Nonneoplastic
AmyloidAngiectasis
Atrophy
Corpora amylacea
Dilatation, cysticb
Dilatation, acinar/vesicle
Fibrosis, stromaa
Infiltrate [insert appropriate cell type]aInflammationa
Metaplasia, squamousb
Necrosisa
Necrosis/inflammation, vascular/perivascularc
Single-cell necrosisa
Vacuolation, epitheliuma
Proliferative (nonneoplastic)
Hyperplasia, atypicalHyperplasia, epitheliumb
Hyperplasia, functionald
Hyperplasia, reactive
Mesenchymal proliferative lesion

CommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

Terminology addressed in Systemic Pathology section.
Terminology with diagnostic criteria or comments described in the text.
cTerminology addressed in the Cardiovascular section.
d
Not a preferred term; refer to the comments for hyperplasia, epithelium.
bComments
Hyperplasia of the epididymal epithelium has been reported inbeagle dogs older than 2 years of age and was most common indogs older than 7 years of age.156 Therefore, spontaneous
epithelial hyperplasia is unlikely to be encountered in the ageof dogs generally used in toxicology studies. Cribriform change
is a change that is more likely to be observed than a true
hyperplasia of the epididymal epithelium.

Diagnostic Features

Focal or lobular “cystic” acini.
Acini may be lined by low cuboidal “atrophic” appearing epithelium or by columnar secretory epithelium.

Differential diagnosesNA.

Comments
Accessory Sex Gland (Prostate)
Table 12.3 summarizes the histopathological lesions that canbe observed in the prostate of beagle dogs in nonclinical toxicological studies.

Dilatation, Cystic—Prostate (Figure 12.7)“Dilatation, cystic” is the preferred term over “dilatation, acinar/vesicle,” which is the term recommended for rodents.139
Dogs have only a prostate and do not have seminal vesicles as
opposed to rodents. Dilated acini are common, generally considered normal, and do not warrant diagnosis. Focal, cystic
dilatation of acini in the canine prostate can be common.144
When observed, the modifier term “cystic” should beconsidered.

Other Term(s)Dilatation, cystic acinar; dilatation, acinar/vesicle (not recommended, see comment).

Metaplasia, Squamous—Prostate(Figures 12.8–12.13)

Pathogenesis

Other Term(s)Origin unknown.

Not applicable.

64

Toxicologic Pathology 49(1)Pathogenesis

Comments

Replacement of the normal columnar epithelium by stratified
squamous epithelium.

“Hyperplasia, epithelium” is the preferred term over“hyperplasia, functional,” which was the term proposed forrodents,139 but is no longer a preferred term. Prostatic hyperplasia has been reported as a common finding in mature beagles
older than the young dogs generally included in routine toxicity
studies.160

Diagnostic Features

Replacement of the normal prostatic glandular columnar
epithelium by multiple layers of flattened epithelial
cells.
Squamous metaplasia may preferentially affect the periurethral glands of the prostate rather than subcapsular
glands.157

Differential Diagnoses
Not applicable.

Comments
Replacement of the prostatic glandular epithelium by stratified
squamous epithelium had been reported following treatment ofdogs with estrogen.157–159 Squamous metaplasia of the prostatic epithelium may also be associated with inflammation.

Hyperplasia, Epithelium—Prostate
Other Term(s)Diffuse hyperplasia.

Modifiers
Diffuse.

Pathogenesis
Proliferation of the epithelial cells lining the prostatic acini.

Diagnostic FeaturesProliferation of acinar epithelium in a single layer thatcan be a diffuse change.
There may be papillary folding of the lining epithelium
into the acinar lumina.
There may be cystic acini lined by flattened epithelium.

Differential Diagnoses

Dilatation, cystic: Focal cystic dilation of acini in theabsence of epithelial hyperplasia elsewhere in theprostate.
Hyperplasia, atypical: A focal hyperplastic lesion withmultilayered epithelium and cellular atypia.
Hyperplasia, reactive: Epithelial hyperplasia accompanied by inflammation.

13. Respiratory System
Introduction
Standardized INHAND nomenclature of proliferative and nonproliferative respiratory tract lesions in rats and mice was previously published. 161 This document follows a similar
anatomical approach but will focus on lesions that are uniqueto the dog, particularly purpose bred beagle dogs commonlyused in research laboratories. Consequently, the nasal cavity,
larynx, trachea, bronchi, bronchioles, lung parenchyma, andpleura have been separated in this nomenclature scheme, eventhough there may be considerable redundancy regarding
responses at various levels. For general considerations of therespiratory system, refer to the published rodent publication.161
The standardized nomenclature for the dog, including respiratory tract lesions, presented in this document is also available
electronically at the STP website3 and in more detail at the
goRENI web site.2 The respiratory system is susceptible toinjury caused by blood borne or inhaled xenobiotics and thus
should be routinely examined in all toxicity studies. While theprimary purpose of this publication is to provide a standardized
nomenclature for classifying microscopic lesions, standardization of tissues sampled and examined is also important, particularly in inhalation studies. The respiratory tract is composedof multiple organs and ancillary tissues including the nasal
cavity, nasopharynx, larynx, trachea, tracheal bifurcation, lung
(7 lobes), and draining lymph nodes. The nasal cavity, larynx,
and lung are complex organs which have multiple types of
epithelia and specific anatomic structures that vary with location. Each of these organs, cells types, and structures must be
individually examined for accurate assessment of the respiratory tract. For inhalation studies, it is recommended that tissues
include 4 transverse levels of nasal cavity, 2 transverse levelsof larynx, 1 or 2 transverse levels of trachea, 1 longitudinal(horizontal) section of carina, samples of all lungs lobes toinclude proximal and peripheral sections, and the tracheobronchial lymph nodes. Tissue sampling for noninhalation studies
may be less extensive but should include at least 2 sections oflung (a proximal area near the main stem bronchi and an area at
the periphery) and 1 section of trachea. One transvers section of
the epiglottis and one transverse section of the oropharynx
bordered by the tonsils may be (optionally) sampled as needed
for specific studies. This sampling is illustrated in the Respiratory Collection Protocol of the Laboratory Dog, published by
GlaxoSmithKline, Charles River Laboratories, and EPL(2006).

Woicke et al.

65

Table 13.1. Microscopic Findings in the Respiratory System: Nasal Cavity, Nasopharynx, and Paranasal Sinuses; Dog.
Nasal cavity, nasopharynx, paranasal sinuses
Congenital
Cleft palatea
Deviation, nasal septum
Nonneoplastic
AmyloidAngiectasis
Atrophy
Congestionb
Corpora amylacea
Degeneration
Edemac,d
Eosinophilic globules
Erosion/ulcer
Hemorrhagec
Infiltrate [insert appropriate cell type]c
Inflammationc
Necrosisc
Perforation, septum
Regeneratione
Thrombusc
Proliferative (nonneoplastic)
Hyperplasia, atypicalHyperplasia, basal cell
Hyperplasia, neuroendocrine cell
Hyperplasia, olfactory epithelium
Hyperplasia, respiratory epithelium
Hyperplasia, squamous cell
Hyperplasia, transitional epithelium
Hyperplasia/metaplasia, mucous cellf
Metaplasia, respiratory, olfactory/glandular epithelium
Metaplasia, squamous cell

CommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

Animals with this change are usually culled and not used in toxicity studies.
Common component of inflammation and described only when it is the predominant feature. May also be associated with incomplete exsanguination and is notdescribed.
c
Terminology addressed in the General Pathology section.
dDescribed if a primary change. Not described if a component of inflammation.
eOften occurs with degeneration. A combined diagnosis (degeneration/regeneration) may be appropriate.
fMucous cell hyperplasia and metaplasia can occur together or independently in the dog nasal cavity (as in the rodent). The pathologist can use either term
independently or together.
bNomenclature
Tables 13.1 to 13.7 summarize the microscopic findings thatcan be observed in the respiratory tract of dogs in preclinical
toxicological studies. Basic terminology is based on the
INHAND Rodent publications, with appropriate modifications
and additions for the dog.

little dorsoventral bend in the nasopharynx (15 ), dogs haveslightly more (45 ), and primates have a marked bend (80 and90 in macaques and humans, respectively).164 In contrast to
mice and rats, which are obligate nose breathers, the anatomyof the nasal and oral cavities of dogs, nonhuman primates, andhumans allows for both nasal and oral breathing.

Nasal Cavity, Nasopharynx, and Paranasal Sinuses

Larynx and Carina

The dog nasal cavity is anatomically more complex thanhumans, has a greater relative surface area (surface/volume),
and has different amounts and distributions of epithelial cell
types.162–164 The greatest difference is in the percentage of thenasal cavity lined by olfactory neuroepithelium. Overall, thedog nasal cavity is less similar to humans than nonhuman primates.165 The degree of bend in the nasopharyngeal airways
impacts local dosimetry of some inhaled agents. Rodents have

The larynx and carina in dogs are anatomically and histologicallymore similar to humans and respond to xenobiotics in a moresimilar fashion to humans than rats.166,167

Bronchi and Bronchioles
Pulmonary conducting airways of dogs are anatomically moresimilar to humans than rats and mice based on the presence (in66

Toxicologic Pathology 49(1)Table 13.2. Microscopic Findings in the Respiratory System: Larynx; Dog.
LarynxNonneoplastic
Degeneration
Ectasia, submucosal glanda
Edemab,cEpithelial alteration
Erosion/ulcer
Hemorrhageb
Infiltrate [insert appropriate cell type]b
Inflammationb
Necrosisb
Regenerationd
Proliferative (nonneoplastic)
Hyperplasia, atypicalHyperplasia, mucous cell
Hyperplasia, neuroendocrine cell
Hyperplasia, respiratory epithelium
Hyperplasia, squamous celle
Metaplasia, squamous cell

CommonUncommonNot observed, but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

Common background finding. Describe only if greater severity/extent than commonly seen as an incidental background finding.
Terminology addressed in the General Pathology section.
cDescribed if a primary change. Not described if a component of inflammation.
dOften occurs with degeneration. A combined diagnosis (degeneration/regeneration) may be appropriate.
eOften accompanied by hyperkeratosis. Hyperkeratosis may be the predominant feature and should be described in the narrative.
b

Table 13.3. Microscopic Findings in the Respiratory System: Trachea; Dog.
Trachea
Nonneoplastic
Degeneration
Ectasia, submucosal glanda
Edemab,c
Erosion/ulcer
Hemorrhageb
Infiltrate [insert appropriate cell type]b
Inflammationa
Necrosisb
Regenerationd
Thrombusb
Proliferative (nonneoplastic)
Hyperplasia, atypicalHyperplasia, mucous cell
Hyperplasia, neuroendocrine cell
Hyperplasia, respiratory epithelium
Hyperplasia, squamous cell
Metaplasia, squamous cell

CommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

Common background finding. Describe only if greater severity/extent than commonly seen as an incidental background finding.
Terminology addressed in the General Pathology section.
Described if a primary change. Not described if a component of inflammation.
dOften occurs with degeneration. A combined diagnosis (degeneration/regeneration) may be appropriate.
bcdogs but not rats or mice) of cartilage and submucosal glands inintrapulmonary bronchi, of mucus cells as the primary secretory cell type in conducting airways, and the presence ofseveral generations of respiratory bronchioles.164 However,tracheobronchial branching patterns of laboratory animals differ from those of humans. Human tracheobronchial airways areWoicke et al.

67

Table 13.4. Microscopic Findings in the Respiratory System: Bronchi; Dog.
BronchiCommonCongenital
Aplasia/hypoplasia, bronchial cartilagea
Nonneoplastic
Bronchiectasis
Degenerationb
Ectasia, submucosal glandc
Edemad,eErosion/ulcer
Hemorrhaged
Infiltrate [insert appropriate cell type]dInflammationd
Necrosisd
Regenerationb,fProliferative (nonneoplastic)
Hyperplasia, mucous cellb
Hyperplasia, neuroendocrine cell
Hyperplasia, respiratory epithelium
Hyperplasia, squamous cell
Hyperplasia, atypical
Metaplasia, squamous cellb

UncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

Terminology with diagnostic criteria and/or comments described in the text.
Finding more frequent as an induced change in inhalation studies.
Common background finding. Describe only if greater severity/extent than commonly seen as an incidental background finding.
dTerminology addressed in the General Pathology section.
eDescribed if a primary change. Not described if a component of inflammation.
f
Often occurs with degeneration. A combined diagnosis (degeneration/regeneration) may be appropriate.
bcTable 13.5. Microscopic Findings in the Respiratory System: Bronchioles; Dog.
Bronchioles
Nonneoplastic
Bronchiectasis
Degenerationa
Edemab,c
Erosion/ulcer
Hemorrhageb
Infiltrate [insert appropriate cell type]b
Inflammationb
Material, extracellular [insert morphology/color]
Necrosisb
Pigment/foreign materiald
Regeneratione
Proliferative (nonneoplastic)
Hyperplasia, mucous cellaHyperplasia, neuroendocrine cell
Hyperplasia, respiratory epithelium
Hyperplasia, squamous cell
Hyperplasia, atypicalHypertrophy/hyperplasia, smooth musclef
Metaplasia, mucous cell
Metaplasia, squamous cellaCommonUncommonNot observed, but potentially relevant

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

Finding more frequent as an induced change in inhalation studies.
Terminology addressed in the General Pathology section.
cDescribed if a primary change. Not described if a component of inflammation.
d
For a description, refer to Pigment/foreign material under Terminal Bronchioles or Alveoli of the rodent manuscript.
eOften occurs with degeneration. A combined diagnosis (degeneration/regeneration) may be appropriate.
fTerminology with diagnostic criteria and/or comments described in the text.
b

Not applicable68

Toxicologic Pathology 49(1)Table 13.6. Microscopic Findings in the Respiratory System: Respiratory Bronchioles, Alveolar Ducts, and Alveoli; Dog.
Respiratory bronchioles, alveolar ducts, alveoli
Congenital
Cyst, congenital
Hypoplasia
Nonneoplastic
Alveolar emphysema
Alveolar lipoproteinosisa
Atelectasis
Congestionb,cDegeneration
Ectasia, acinus
Edemab,d
Embolus
Fibrosis/epithelial hyperplasiae
Fibrosisb,f,gHemorrhageb,g,hHypertrophy, media, arteries/arterioles
Infiltrate [insert appropriate cell type]b
Inflammationb
Macrophages increasede,iMaterial, extracellular [insert morphology/color
Mineralizationb
Necrosisb
Pigment/foreign materialb,jRegeneration
Thrombusb,kProliferative (nonneoplastic)
Cyst, keratinizing
Hyperplasia, bronchioloalveolar
Metaplasia, mucous cell
Metaplasia, osseouse
Metaplasia, squamous cell

CommonUncommonNot observed but potentially relevant

Not applicable
x

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

a

Diagnosis based on electron microscopy or immunohistochemistry.
Terminology addressed in the General Pathology section.
cCommon component of inflammation and described only when it is the predominant feature. May also be associated with incomplete exsanguination and is notdescribed.
dDescribed if a primary change. Not described if a component of inflammation.
eTerminology with diagnostic criteria and/or comments described in the text.
fCommon background finding as a minimal, focal, or multifocal change. Describe only if greater severity/extent than commonly seen as an incidental backgroundfinding.
g
Suggested preferred terms: Fibrosis followed by locator or hemorrhage, followed by locator term.
hAlveolar hemorrhage is frequently accompanied by hemoglobin crystals and low numbers of macrophages and/or leukocytes (especially neutrophils). These are acomponent of hemorrhage and should not be recorded as separate findings.
i
Use of the term “macrophage aggregation” is not recommended and was replaced by “macrophages increased”.
jPigments such as anthracosis and hemosiderin are common background finding and should be described only if extensive.
kMay be seen in large or small vessels including capillaries. Thromboemboli may be common in chronic infusion studies.
bdichotomous, that is, they are relatively symmetrical anddaughter branches are of relatively equal diameter. In contrast,
tracheobronchial airways of monkeys, rodents, and dogs aremonopodial, that is, daughter branches are of unequal diameterand the smaller airway branches at a different angle from thelarger airway. Humans and rodents have more generations ofnonrespiratory bronchioles than dogs and monkeys. Club cells
are present mainly in terminal and respiratory bronchioles ofhumans, monkeys, and dogs, but dog club cells are distinctive
due to the large amount of glycogen in their cytoplasm. Overall, the epithelial types and distribution of secretory cells(mucus, serous, and club cells) lining airways are more similar
in dogs to humans than those of rats or mice.168

Aplasia/Hypoplasia Bronchial Cartilage—
Lung–Bronchi
Pathogenesis/Cell of Origin
Congenital change. Aplasia, hypoplasia or dysplasia of bronchial cartilage.

Woicke et al.

69

Table 13.7. Microscopic Findings in the Respiratory System: Pleura; Dog.
PleuraCommonCongenital
Cyst, congenital
Nonneoplastic
Effusion, noninflammatory
Fibrosisa
Hemorrhagea
Infiltrate [insert appropriate cell type]aInflammationa
Mineralizationa
Pigment/foreign materiala
Pyothorax
Proliferative (nonneoplastic)
Hyperplasia, mesotheliumaUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x

Terminology addressed in the General Pathology section.

Diagnostic Features







Lack of or incomplete development of the cartilage ofthe bronchial tree.
Absent, incomplete, rudimentary cartilaginous ringaround bronchi.
Cartilage may be disorganized.
Bronchial glands and muscle fiber are present.
Bronchi are lined by a cuboidal to columnar epithelium
which may be immature.
May be associated with alveolar emphysema, dilated
bronchioles, and terminal airways.

Comment
This is a rare change that has been reported in nonlaboratory
dogs.169–171 The change is often associated with dyspnea.
Laboratory dogs with this change would likely be culled prior
to being used in toxicity studies.

May be associated with an adjacent inflammatory cell
infiltrate.
No disruption, compression, or disturbance of adjacenttissues.

Differential DiagnosesTangential section through vessel: Other structures in section
are cut tangentially, missing, or otherwise abnormal in shape.

Comment
Smooth muscle cell hypertrophy/hyperplasia is uncommon as aspontaneous finding but can be seen in cases of lung inflammation, allergic disease, and/or immune stimulation, especially following inhalation administration of certain xenobiotics.172 Often
it is observed as a focal/multifocal change rather than diffusely
present throughout all areas of the parenchyma.

Fibrosis/Epithelial Hyperplasia—Lung:
Alveoli (Figure 13.1 and 13.2)
Hypertrophy/Hyperplasia, Smooth Muscle—
Lung: Bronchi/Bronchioles

Other Term(s)Pathogenesis/Cell of Origin

Fibrosing alveolitis, segmental fibrosis, segmental subpleural
septal fibrosis; these terms are no longer preferred.

Smooth muscle cells in the submucosa of bronchi andbronchioles.

Pathogenesis/Cell of Origin
Fibroblasts, bone marrow-derived inflammatory cells. Usuallyaccompanied by epithelial hyperplasia.

Diagnostic Features

Increased prominence of smooth muscle mass characterized by enlarged smooth muscle cells with or without
increased nuclear density.
Smooth muscle cells are well differentiated and aligned
appropriately.
Usually the thickened muscle layer is evenly distributed
around the airways (depends on section).

Diagnostic FeaturesAreas of alveolar septal thickening due to fibrosis andinterstitial inflammation. Thickened septa are typicallylined by hyperplastic epithelium.
Subpleural location, extending from pleura into pulmonary parenchyma. May be wedge-shaped with pleuralfibrosis.

70

Toxicologic Pathology 49(1)Increased alveolar macrophages and squamous metaplasia may be present.
The predominance of inflammation and epithelial
hyperplasia may vary with lesion age such that fibrosis
and inflammation are the main features in some lesions
whereas fibrosis and epithelial hyperplasia predominate
in others.

alveolar phospholipidosis is not a synonym for increased alveolar macrophages and should be reserved for accumulations ofphospholipid-containing macrophages observed after treatment
with cationic amphiphilic drugs.175,176

Metaplasia, Osseous—Lung: Alveoli

Comment

Other Term(s)Common background finding in beagle dogs. Unique to dogs.
Differs from pulmonary fibrosis in its consistent subpleurallocation and characteristic alveolar septal fibrosis with epithelial hyperplasia. A high incidence of this finding, unrelated totest article administration, may be misinterpreted by pathologists unfamiliar with background pathology in laboratory beagles when it occurs in treated animals.173 Possibly related to old
infections with Filaroides or Toxicara species.60

Ectopic bone, ectopic ossification, pulmonary ossification, pulmonary osteophytes.

Pathogenesis/Cell of Origin
Transformation of differentiated mesenchymal cells into bonecells.

Diagnostic Features
Macrophages, Increased—Lung: Alveoli(Figure 13.3 and 13.4)
Other Term(s)Alveolar macrophage aggregation, alveolar macrophage accumulation, alveolar histiocytosis.

Pathogenesis/Cell of Origin
Bone marrow-derived monocyte precursors; resident interstitial and alveolar macrophages proliferate locally in adults.

Diagnostic Features

Increased numbers of alveolar macrophages over andabove the expected number of randomly scattered individual macrophages within the alveolar spaces.
Often aggregated in multiple foci. However, some of theapparent aggregation and location may be an artifact of
intratracheal instillation of fixative.

Comment
The term increased alveolar macrophages is intended to replace
alveolar macrophage aggregation in the rat INHAND document, particularly for inhalation studies.174 It is preferable touse increased alveolar macrophages as a stand-alone diagnosis
and not as an umbrella term that includes other inflammatory,degenerative, or proliferative responses. The term also pertains
to lesser numbers of interstitial macrophages that might accompany increased alveolar macrophages. The affected compartment should be addressed in the report narrative. Increased
interstitial macrophages should only be diagnosed separately
if the pulmonary interstitium represents the predominant location. If the macrophages from treated animals have cytoplasmicchanges (pigment, vacuolation, foreign material, etc) that are
distinct from features in controls (historical or current), thesechanges should be part of the diagnostic term. The termNodule(s) of bone tissue within the parenchymal
interstitium.
Bone marrow may be present.

Differential Diagnoses
Mineralization: No differentiated bone tissue, that is, no cellular component.

Comment
Osseous metaplasia of the lung is an uncommon incidental
finding in dogs.49,60 The change is similar to that seen inrats but occurs at a lower incidence in dogs than in rats.

14. Skeletal System and Tooth
Introduction
For detailed general considerations of the skeletal system,
please refer to the rodent publication. While the type and classification of bone and joints lesions is identical or very similar
across species, their frequency in untreated individuals may
differ substantially in the various species.
The standard bone samples examined during a toxicity study
in dogs varies among institutions; they usually include a section of sternum or rib and a section of a long appendicular bonewith the articular surface. The selected long bone should betrimmed longitudinally to include a section of the articularcartilage, epiphysis, growth plate, metaphysis, and corticalbone in the diaphysis. The distal femur with or without theproximal tibia, the femorotibial joint, or the proximal tibia aregood options for evaluation of long bone in dogs. In standard
toxicity studies, bone tissue is decalcified before it is subjectedto the usual processing in the histology laboratory. Undecalcified sections may be prepared if the cortical remodeling, boneformation rate, and/or status of mineralization needs to beaddressed.

Woicke et al.

71

Table 14.1. Microscopic Findings in the Skeletal System: Bone; Dog.
Bone
Nonneoplastic
Bone decreased, trabeculae and/or cortex
Bone increased, trabeculae and/or cortexa
Cyst, bone
Eroded surface, increased
Fibro-osseous lesion (FOL)
Fibrous dysplasiaa
Fibrous osteodystrophy (FOD)
Fracture
Growth plate closed
Growth plate closed/partially closed/opena
Necrosis
Osteoblastic surface, increased
Osteoclasts, increased
Osteoid, increased
Physeal dysplasia
Physis, decreased thickness,
Physis, increased thickness,
Proliferative (nonneoplastic)
Hyperplasia, chondrocyte
Hyperplasia, osteoblast, focalaCommonUncommonNot observed
but potentially relevantNot
applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

Terminology with diagnostic criteria and/or comments described in the text.

The discussion here is focused on spontaneous lesions
observed in young dogs (up to 2 years of age) used in preclinicalstudies. The assignment to the categories “common” and“uncommon” is subjective to a certain extent and refers to theincidence of lesions in untreated beagle dogs used in toxicity
studies. Generally, nonproliferative skeletal abnormalities areuncommon spontaneous findings in young dogs and proliferative
neoplastic lesions are rarely observed as background findings inroutine toxicity studies due to the young age of animals and theshort duration of studies compared to the dog’s natural life span.
Fractures in the bones are very rarely observed and joint
lesions, more frequently seen in older dogs with a predilectionin certain breeds, are also uncommon spontaneous findings in
untreated young beagle dogs.

Bone
The recommended nomenclature of nonneoplastic and neoplastic bone changes in the dog is based on the INHAND manuscript for rodents and the online publication of the goReni
website. Comments specific to the dog were added to the terms
increased bone, trabeculae, and/or cortex and osteosarcoma.
Terms added to the dog nomenclature that were not considered
pertinent to the rodent included bone increased, trabeculae
and/or cortex, fibrous dysplasia, and growth plate closed/partially closed/open. The recommended nomenclature is presented in Table 14.1. For a description of the neoplasticchanges not pertinent to rodents but potentially to dogs, referto the Histological Classification of Bone and Joint Tumors ofDomestic Animals published by the Armed Forces Institute of

Pathology in conjunction with the American Registry of
Pathology and the World Health Organization,177 the textbook
Tumors of Domestic Animals,178 and /or current literature.
Genetic and congenital diseases were considered out of scope
for this tabulation.

Bone, Increased, Trabeculae, and/or
Cortex—Bone
Comment
In the dog, there are a number of conditions that are characterized by excessive bone formation, “hyperostosis,” including
craniomandibular osteopathy, hypertrophic osteopathy (HO),panosteitis, and canine hepatozoonosis. While craniomandibular osteopathy and panosteitis (also referred to as enostosis,
eosinophilic panostitis, juvenile osteomyelitis, or fibrous osteodystrophy) have been described in other breeds than the beagle
and canine hepatozoonosis infection should not occur in indoor kept animal colonies, HO could theoretically affect abeagle dog. Typically, however, it is observed in large breeds
and boxers; to the author’s knowledge it has not been observedin toxicity studies with beagle dogs. Salyusarenko et al179 characterized HO as pathologic periosteal bone formation, which isobserved predominantly in the metacarpi or metatarsi (76%),
distal and proximal long bones (66% and 38% respectively),
carpi and tarsi (31%), and phalanges (21%). Hypertrophic
osteopathy is described to occur secondary to other conditions,mostly pulmonary or intrathoracic neoplasms.179,180 Dogs withprimary lung neoplasia or osteosarcoma metastases in the lungs
appear to be most often affected. The pathogenesis of HO is not

72

Toxicologic Pathology 49(1)known, a number of theories exist, increased limb blood flow isa consistent early change, and induction of vascular endothelial
growth factor via cyclo-oxygenase-2 derived increase in prostaglandin E2 appears to be involved.181–183 Changes accompanying increased bone, for example, inflammation, should beeither recorded separately or their occurrence and nature could
be mentioned in the narrative of the report.

Pathogenesis/Cell of Origin
Ossification of the growth plate

Diagnostic Features

Fibrous Dysplasia—BoneThickness of the physis depends on the rate of growth.
When growth is slowing, the layers of the physis narrow.
At the end of the process, the cartilage of the growthplate is replaced by a bony scar, which is remodeled intotrabecular bone.

Other Term(s)Differential Diagnoses

Not applicable.

Not applicable.

Pathogenesis/Cell of Origin

Comment

Disorder of development of bone, associated with mutations of
the GNAS1 gene184; all the components of normal bone arepresent but they do not differentiate into their mature structures(ie, no rimming of trabeculae by osteoblasts and no evidence oflamellar replacement of woven bone).

Unlike the rodent, the physis of dogs closes when the animal
reaches maturity and growth ceases. Usually, it is not considered necessary to record the status of physis closure in a standard toxicity study. However, the recording of physis closure
may become necessary if a treatment causes or is considered topossibly cause a delay in physis closure, a premature physis
closure, or maturity of other organs (eg, male and female genital tract organs) have to be compared against skeletal maturity.
In the untreated, healthy dog, growth plate closure of tibia
and femur occurs at the age of 6 to 11 months26; the closure of
the distal femur physis in the beagle occurs between 250 and 325
days of age (8-11 months) in females slightly earlier than inmales.188 Similar data of epiphyseal closure of the distal femur
are given by Zoetis et al189; their publication provides a detailed
survey of skeletal maturation and postnatal bone growth offemur and humerus across species including humans and dogs.

Diagnostic Features







Well-circumscribed intramedullary lesions that can
expand and distort the bone.
Fibrous dysplasia can involve a single bone (monostotic) or several bones (polystotic).
Characterized by disorganized trabeculae of wovenbone that lacks conspicuous osteoblastic rimming, surrounded by a moderately cellular fibroblastic
proliferation.
Nodules of hyaline cartilage may be present.
Cystic degeneration, hemorrhage, and foamy macrophages can occur.

Differential Diagnoses

Fibrous osteodystrophy: generalized condition; osteoblastic rimming present.
Ossifying fibroma: localization in the jaws; presence ofosteoblastic rimming around trabeculae.

Comments
Fibrous dysplasia has been reported in a number of species,
including the dog, and is usually observed as a solitary lesion
but also has been reported in animals and man in the polyostotic form. It is a rare, nonaggressive, radiolucent, nonneoplastic lesion considered developmental in origin.185–187

Joint
The recommended descriptive nomenclature for lesions in the
joint of dogs is summarized in Table 14.2, which is based on the
INHAND rodent nomenclature. It should be noted, however,that in beagle dogs used in toxicity studies, spontaneous lesionsof the joints are rare, and neoplastic joint lesions are extremely
rare. For a histomorphologic description of neoplasms not pertinent to rodents but potentially to dogs, refer to the WHO fascicle
on Histological Classification of Bone and Joint Tumors ofDomestic Animals,177 the chapter on joint tumors190 in the textbook Tumors of Domestic Animals, and/or current literature.

Degeneration, Articular Cartilage—Joint
Other Term(s)Growth Plate Closed, Partially Closed,
Open—Bone
Other Term(s)Physis closed, partially closed, open.

Arthropathy

Pathogenesis/Cell of Origin
Articular cartilage; degenerative changes due to administrationof xenobiotics.

Woicke et al.

73

Table 14.2. Microscopic Findings in the Skeletal System: Joint; Dog.
Joint

CommonNonneoplastic
Degeneration, articular cartilagea,bDegeneration, chondromucinous
Degenerative joint disease (DJD)
Inflammationc
Osteophyte
Proliferative (nonneoplastic)
Hyperplasia, synovial cell

UncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x

a

Terminology with diagnostic criteria and/or comments described in the text.
Finding more frequent as an induced change.
cTerminology addressed in Systemic Pathology section.
b

Diagnostic Features









Vesicle and/or fissure formation in the articularcartilage.
Accumulation of fluid in vesicles, accumulation of debris within fissures (eg, collagen fibrils, eosinophilicmaterial, granular extracellular matrix material).
Spindle-shaped cells adjacent to the fissures.
Degenerative changes in chondrocytes, for example,
shrunken cytoplasm; shrunken, pyknotic nuclei; staining
abnormalities such as poor staining of chondrocytes;
dark purple matrix or lighter purple matrix (toluidine
blue stain).
Flap formation or loss of superficial cartilage.
Clusters of chondrocytes, indicating regeneration.

rodent nomenclature. Added terminology for the dog is limited
to inflammation, periodontal (periodontal disease), which represents the most common dental disease in dogs, although it israrely observed in young beagles used in toxicity studies. For
histomorphologic criteria of dental neoplasms, refer to the WHO
fascicle on Histological Classification of Tumors of the Alimentary System of Domestic Animals,24 the chapter on tumors of thealimentary tract25 in the textbook Tumors of Domestic Animals,
and/or current literature.

Inflammation, Periodontal—Tooth
Other Term(s)Gingivitis
Periodontal disease

Differential Diagnoses
Spontaneous degeneration; occurs in older dogs.

PathogenesisComment
Quinolone arthropathy represents a typical example of druginduced joint degeneration affecting major synovial joints; it
has been observed in many species including the dog.191–193




Tooth
Teeth are not a part of the routine histopathological evaluation in
dog toxicity studies but may be added to the spectrum of organs
to be examined if considered necessary, for instance, if they are
the subject of investigation. Dogs, in contrast to rodents, arediphyodont with deciduous and permanent dentition. Shabestari
et al194 describe the dental eruption time specifically in in beagle
dogs; the deciduous dental formula is I 3/3, C1/1, M 3/3; thepermanent dental formula is I3/3, C1/1, PM 4/4, M 2/3. Tooth
lesions are uncommon in beagle dogs,195 in part because theteeth are rarely examined and, in toxicity studies, the dogs areof young age. The incisor, canine, or mandibular tooth can be
decalcified and evaluated if needed.
The recommended descriptive nomenclature for tooth lesions
is presented in Table 14.3, which is based on the INHAND

The process is usually initiated by plaque (tartar) accumulation and starts as acute inflammation (increased
number of neutrophils and increased fluid); later on,
plasma cell and macrophage accumulation follow.
Loss of gingival collagen fibers, caused by cytokines
and enzymes of the plaque bacteria.
Apical recession of the tooth–gingiva junction andresorption of alveolar bone.
Destruction of the periodontium and periodontal
ligament.
Resorption of alveolar bone, cementum, and root dentin
lead to exfoliation of teeth.

Diagnostic Features
The diagnosis is in most instances readily made grossly (plaqueformation, reddening, and swelling of the gingiva). Microscopically, it is characterized by the inflammatory process and theassociated inflammatory cell infiltrates.

15. Soft Tissue and Skeletal Muscle
For general aspects of soft tissue and muscle lesions, pleaserefer to the rodent manuscript.196 Soft tissue and skeletal

74

Toxicologic Pathology 49(1)Table 14.3. Microscopic Findings in the Skeletal System: Tooth; Dog.
Tooth

CommonCongenital
Denticle
Nonneoplastic
Cyst
Degeneration
Dental dysplasia
Dentin matrix alteration
Dentin niches
Dentin, decreased
Fracture
Inflammation, periodontala
Necrosis
Periodontal pocket
Pulp concretion
Resorption
Thrombusab

UncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x

Terminology with diagnostic criteria and/or comments described in the text.
Finding more frequent as an induced change.

muscle lesions observed in the dog are generally identical tothose observed in rodents and other domestic animals as well as
those in humans.196 Therefore, the nomenclature of nonneoplastic and neoplastic soft tissue and skeletal–muscle changes
described in the INHAND manuscript for rodents and the structure of the rodent classification were adopted for the dog.
Included are lesions of the soft and adipose tissue, smooth andskeletal muscle, and the mesothelium. Lesions of the synovium
are described in the chapter on the skeletal system. For detailed
general considerations on the soft tissue and skeletal muscle,
please refer to the rodent publication.
While the type and classification of soft tissue lesions is
identical or very similar across species, their frequency inuntreated individuals may differ substantially in the various
species. The assignments to the categories “common” and“uncommon” are subjective to a certain extent and refer to theincidence of lesions in untreated beagle dogs used in toxicity
studies. The incidence may increase significantly under certain
treatment regimens, such as granuloma formation observed in
the subcutaneous connective tissue of dogs after the implantation of microchips197 or skeletal muscle hypertrophy following
inhibition or blockade of myostatin gene.198

Skeletal Muscle
The special cellular structure of skeletal muscle, the different fiber types, and their varying sensitivity in the response
to xenobiotics require a separate consideration of the pathological changes and also the extent of sampling and carefulselection of sampling sites.196 Skeletal muscle fiber types
are characterized by the histochemical reaction for adenosine triphosphate and the pH lability of this reaction.199,200
Species-specific type II subtypes have been identified forthe dog: II A, II dog (unique to the dog), and II C.201 An
investigation based on reverse transcription polymerase
chain reaction and confirmatory histochemistry, immune
histochemistry, and sodium dodecyl sulfate polyacrylamide
gel electrophoresis showed that limb and trunk muscles ofdogs express myosin heavy chain (MHC) types 1, 2A, 2X,
and that the so-called type 2 dog fibers express MHC-2X.
MHC-2B type fibers were found in extraocular and laryngeal muscles.202
Table 15.3 summarizes the lesions observed in the skeletal
muscle followed by changes in the mesothelium (Table 15.4).
The nomenclature applies to rodents and other laboratory animal species including the dog.

Soft Tissue and Adipose Tissue
The following tabulations (Tables 15.1 and 15.2) summarize
the nonneoplastic lesions observed in the soft and adipose tissue and the smooth muscle of dogs. The terminology is basedon the INHAND rodent terminology, and modifications oradditions for the laboratory beagle dog were not considerednecessary or adequate. As noted in the rodent manuscript, thecareful recording of the nature, intensity, and duration of theinflammatory response in soft tissues to implants or injected
substances is important for the assessment of their local
tolerability.196

Regeneration—Skeletal Muscle
Other Term(s)Not applicable.

Pathogenesis/Cell of Origin
Repair process following muscle injury

Woicke et al.

75

Table 15.1. Microscopic Findings in the Soft Tissue: General Terminology; Dog.
Soft tissue

CommonNonneoplastic
Amyloid
Fibroplasia
Fibrosisa
Infiltrate [insert appropriate cell type]aInflammationa
Metaplasia
Mineralizationa
Necrosisa
Proliferative (nonneoplastic)
Hyperplasia, smooth muscleaUncommonNot observed but potentially relevant

Not applicableNot observed but potentially relevant

Not applicableNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x

Terminology addressed in the General Pathology section.

Table 15.2. Microscopic Findings in the Soft Tissue: Adipose Tissue; Dog.
Adipose tissue

CommonNonneoplastic
Atrophy
Infiltrate [insert appropriate cell type]aInflammationa
Inflammation, lipogranulomatous
Necrosisa
Nonneoplastic proliferative lesions
Hyperplasia, adipose tissueaUncommon
x

x
x
x
x
x

Terminology addressed in the General Pathology section.

Table 15.3. Microscopic Findings in the Soft Tissue: Skeletal Muscle; Dog.
Skeletal muscle
Nonneoplastic
AtrophyDegeneration
Hypertrophy
Infiltrate [insert appropriate cell type]aInflammationa
Mineralizationa
Necrosisa
Regenerationb
VacuolationaabCommonUncommon
x
x
x

x
x
x
x
x
x

Terminology addressed in the General Pathology section.
Terminology with diagnostic criteria and/or comments described in the text.

Diagnostic Features






Increased basophilia
Centrally located nuclei
Chains of nuclei
Disorganized appearance of muscle fibers in the case oftissue disruption
Muscle giant cells in the case of tissue disruption, indicative of abortive regeneration

Differential Diagnoses
Not applicable.

Comment
Skeletal muscle has a high regenerative potential. The processof skeletal muscle regeneration can be divided into 3 phases:
inflammatory, satellite cell activation/differentiation, and76

Toxicologic Pathology 49(1)Table 15.4. Microscopic Findings in the Soft Tissue: Mesothelium; Dog.
Mesothelium

CommonUncommon

Proliferative (nonneoplastic)
Hyperplasia

Not observed but potentially relevant

Not applicable

x

Table 15.5. Microscopic Findings in the Soft Tissue: Relevant Neoplastic Proliferative Lesions; Dog.
Soft tissue: Relevant neoplastic proliferative lesions

CommonHistiocytoma, canine, cutaneousa
Lipomabab

UncommonNot observed but potentially relevant

Not applicablexx

Terminology with diagnostic criteria and/or comments described in the text.
For a histomorphologic description of lipoma, refer to the rodent manuscript, the WHO fascicle on mesenchymal tumors of the skin and soft tissues, the textbook
Tumors of Domestic Animals1, and/or current literature.

maturation.203 The inflammatory phase follows muscle injury
and is characterized by initial infiltrates of neutrophils, followedby pro-inflammatory macrophages and anti-inflammatory
macrophages, which are essential for the activation of satellite
cells. Satellite cells are considered the myogenic stem cells; uponactivation, they divide asymmetrically into new stem cells andmyogenic progenitor cells (2nd phase). Eventually, the progenitor
cells fuse into multinucleated myofibers with centrally locatednuclei. During maturation (3rd phase), the nuclei move toward the
periphery.203,204

Mesothelium
In addition to the general terminology of soft-tissue lesions,findings unique to the mesothelium are limited to hyperplasia(Table 15.4).

Diagnostic Features









Grossly, a dome-shaped protrusion of the skin, hairless,often abraded, and ulcerated.
Dermal (and epidermal) infiltrate of densely packed,
mildly pleomorphic round cells, arranged in cords andnests.
Histiocytic-like neoplastic cells, ovoid, bean-shaped, or
convoluted nuclei, lightly eosinophilic cytoplasm.
Often numerous mitotic figures.
Typically, wedge shaped, that is, narrow basis and wider
near the epidermis,95 “top heavy” epidermal focus.27,96
Little or no stroma.
Immunohistochemistry provides the definitive
proof of the diagnosis97; canine cutaneous histocytomas are considered a Langerhans cell tumor andexpress CD1a, MHC II, CD11c, CD18, and often
E-cadherin.96,98

Neoplastic Proliferative Lesions
In the experience of the authors, the cutaneous canine histiocytoma and the lipoma of the serosa represent relevant
neoplastic findings, that is, these have been noted in the soft
tissues in toxicologic pathology study in dogs; they arepresented in Table 15.5. For a histomorphologic descriptionof neoplasms, refer to the WHO fascicle on mesenchymal
tumors of the skin and soft tissues,95 the chapter on soft
tissue neoplasms99 in the textbook Tumors of Domestic Animals, and/or current literature.

Histiocytoma, Canine Cutaneous—Skin(Figures 8.1–8.3; Section on Integument)
Other Term(s)Not applicable.

Pathogenesis/Cell of Origin
Langerhans cells.

Differential DiagnosesEpitheliotropic T-cell lymphoma: When considered
because of epithelial involvement, differentiation is best
done by IHC.
Plasmacytoma: Sheets of round cells, in nests or poorly
defined cords, pleomorphic nuclei, mono-, bi-, or multinucleated, no or few mitoses, scant to moderate eosinophilic or basophilic cytoplasm, amyloid may bepresent. The IHC may be required for definite differential diagnosis; MUM1 is considered the most reliable
marker.99
Cutaneous histiocytosis: Coalescence of perivascular
infiltrates in the subcutis and deep dermis without involvement of epidermis.

Comment
Usually a solitary lesion of young dogs, less than 3 years old,but has been observed in dogs of all ages. The lesion occurs inall breeds, predisposed are brachycephalic breeds, including

Woicke et al.
boxers, but also dachshund, Doberman pinschers, and cocker
spaniels. Spontaneous regression is a feature.96,100

16. Special Senses—Eye and Glands
Introduction
For general aspects of special senses organs, refer to the rodent
manuscript. The suggested standardized nomenclature for theocular system is subdivided into the eye and the glands of theeye. The ear is not examined routinely in toxicologic pathology
studies and is not included. Recommended nomenclature formicroscopic lesions in the nose is addressed in the respiratory
chapter.

Eye
The eye is a complex organ, and comprehensive review of the
toxicological response of each component is beyond the scopeof this manuscript. In brief, however, the eye is composed oftissues of ectodermal, neuroectodermal, and mesenchymal origin; therefore, abnormalities in development of any of them
may contribute to the abnormal development of adjacent,sequentially developed tissue. Thus, this organ is particularly
sensitive to in utero, as well as postnatal, toxic and teratogenic
insults. In addition, the avascular nature of the lens and cornea
and their resultant dependence on the aqueous humour for
nutrition, as well as the immunoprivileged nature of the eyeitself, are all considerations in the analysis of xenobioticrelated toxicity of the eye. Extraorbital glandular tissue, eyelids, muscles, and other adnexa develop independently of the
globe, although toxic damage to these tissues may affect thehealth of the eye, making evaluation of the adnexa an importantcomponent of toxicity studies. (For example, cytotoxicity in the
lacrimal gland may result in decreased tear production withsubsequent damage to the cornea.) Systemic or topical administration of a drug or compound may result in toxicologically
relevant ocular exposure, potentially compromising not just thestructural but also the functional competency of this organ, so
microscopic evaluation of the eye should generally be supplemented with clinical and ophthalmologic examination for complete analysis. In addition, the intimate association between theretina and the central nervous system may result in changes
within the globe following exposure to certain neurotoxicants.
At necropsy, the eye should be enucleated and placed in an
appropriate fixative immediately after euthanasia. Davidson’s
fluid is most commonly recommended for fixation of the eye,as this fixative is particularly good for preservation of retinalarchitecture. Formalin and Bouin’s fixative have also beenused in the past, but formalin fixation is not optimum for preservation of retinal detail, and Bouin’s fixative contains picricacid and provides a health and safety hazard, if not handledcorrectly. Some pathologists prefer injection of a small amountof fixative into the globe in addition to immersion fixation;
however, for routine toxicology studies, this method is usuallynot necessary. Paraffin-embedded, H&E-stained sections of

77eye, trimmed in a sagittal orientation at the midline of the eye
to capture the optic nerve/optic cup, are usually adequate forroutine histological examination.
Adnexal lacrimal glands are present in the dog in both theorbit and within the third eyelid, referred to in the latter location as the gland of the third eyelid or the nictitans gland. Please
note that Harderian glands, as recognized in the rodent, are notpresent in dogs. Also note that the tapetum lucidum, a reflective layer of the choroid, is well developed in the canine eye
and is generally viewed in routine vertically oriented histologic
sections.
The recommended nomenclature of lesions in the eye ofdogs is presented in Table 16.1.

Adhesion, Iris—Eye: Uvea, Iris, and Ciliary
Body
Comment
While recognized uncommonly as a congenital finding inyoung beagle dogs, adhesion between the anterior edge of theiris and the endothelium of the cornea (anterior synechia) or theposterior edge of the iris and the anterior surface of the lens(posterior synechia) may also be acquired, most commonly as asequela to anterior uveitis, trauma, or other inflammatory
processes.

Infiltrate, Mixed Cell, Conjunctiva—Eye:Conjunctiva (Figure 16.1)
Comment
Within the conjunctiva, mononuclear or lymphoid cell infiltrates may not be indicative of an inflammatory process but
may instead represent CALT, a normal structure of the conjunctival mucosa, particularly prevalent on the bulbar surfaceof the third eyelid.205,206 With chronic antigenic stimulation,
lymphoid hyperplasia may occur in CALT structures.

Cyst—Eye: Uvea, Iris, and Ciliary Body
Other Term(s)Not applicable.

Diagnostic Features

Fluid-filled structure lined by 1 to 2 layers of variably
pigmented cuboidal uveal epithelial cells supported by athin fibrous capsule.
Cysts may be ruptured during processing.

Differential Diagnosis
Uveal melanoma: not usually cystic. Commonly comprised ofplump pigmented cells often with lesser numbers of spindloid
cells.

78

Toxicologic Pathology 49(1)Table 16.1. Microscopic Findings in Special Sense Organs: Eye; Dog.
Eye
Congenital
Filtration angle/trabecular meshwork
Malformation, filtration angle
Uvea, iris, and ciliary bodyAdhesion, irisa
Hypoplasia, ciliary body (iris and ciliary body)
Malformation, iris
Persistent pupillary membrane
Vitreous
Aplasia, vitreous
Persistent hyaloid vessels
Persistent hyperplastic primary vitreous
RetinaRetinal rosettes
Retinal folds
Nonneoplastic
General terms
Apoptosisb
Atrophy
Fibroplasia
Fibrosisb
Infiltrate [insert appropriate cell type]b
Inflammationb
Mitotic figures, increased
Neovascularization
Single-cell necrosisb
EyelidAtrophy, Meibomian gland
Infiltrate [insert appropriate cell type], eyelidb
Infiltrate [insert appropriate cell type], Meibomian glandb
Inflammation, eyelidb
Inflammation, granulomatous, Meibomian glandb
Inflammation, Meibomian glandb
Cornea and conjunctiva
Atrophy, epithelium
Attenuation, endothelium (cornea)
Cyst, inclusion
Edemab
Erosion/ulcer
Fibroplasia
Fibrosis, stromab
Hypertrophy, Descemet membrane
Infiltrate [insert appropriate cell type], conjunctivaa,bInflammationb
Keratinization (cornea)
Mineralization (cornea)b
Neovascularization (cornea)
Pigmentb
Vacuolation, epithelium or endothelium (cornea)b
Filtration angle/trabecular meshwork
Narrowed filtration angle
Inflammation (anterior chamber, aqueous humor)
Proteinous fluid (anterior chamber, aqueous humor)
Uvea, iris, ciliary body, choroid
Atrophy, ciliary body
Atrophy, iris
Congestion
Cysta

Common Uncommon Not observed, but potentially relevant Not applicable

x
x
x
x
x
x
x
x
x
x

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
(continued)

Woicke et al.

79

Table 16.1. (continued)
Eye

Common Uncommon Not observed, but potentially relevant Not applicableb

Infiltrate [insert appropriate cell type]
Inflammationb
Neovascularization
Pigment, increased/decreased, irisb
Vacuolation, cytoplasmic, epitheliumb,cLens
Degeneration, lens fiberd
Dislocation, lens, anterior or posteriord
Fibroplasia, lens epithelium
Hypertrophy, lens capsule
Hypertrophy, lens epithelium
Hypertrophy, lens fiber
Inflammation, lensb
Mineralization, lens fiberb
Necrosis, lens epithelium
Rupture, lens capsuleb,dVacuolation, lens epithelium or lens fiber
Vitreous
Fibroplasia; vitreous fibrosis
Hemorrhageb
Infiltrate [insert appropriate cell type]b
Inflammationb
Metaplasia, bone or cartilage
Mineralization, vitreousb
Pigment, macrophage, hemosiderin
RetinaArteriolar loop, pre-retinal (retina)
Atrophy (retinal pigment epithelium)
Atrophy, inner retina
Atrophy, outer retinac
Atrophy, retina, global
Degeneration, microcystoida
Degeneration, tapetum lucidum (choroid)a,c
Detachment, retinad
Displacement, photoreceptor nuclei
Fibroplasia, retinal or epiretinal
Glial cells, increased number
Hemorrhage (retina)b
Infiltrate [insert appropriate cell type]b
Inflammationb
Mineralization
Myelin, increased
Neovascularization
Pigment, increased
Single-cell necrosisb
Vacuolation, cytoplasmb,cVacuolation, extracellularb
Optic nerve
AtrophyDegeneration, axon
Demyelination
Glial cells, increased number
Infiltrate [insert appropriate cell type]b
Inflammationb
Vacuolationb,dRetinal pigment epithelium (RPE)
Atrophy
Deposits, extracellular matrix, subretina

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
(continued)

80

Toxicologic Pathology 49(1)Table 16.1. (continued)
Eye

Common Uncommon Not observed, but potentially relevant Not applicable

Fibroplasia, subretina
Hypertrophy, RPE
Inclusions (intracytoplasmic accumulation), RPEc
Necrosisb
Pigmentation, decreasedb
Pigmentation, increasedb
Polarity, loss, RPE
ScleraAtrophy
Infiltrate [insert appropriate cell type]b
Inflammationb
Metaplasia, bone or cartilage
Proliferative (nonneoplastic)
EyelidHyperplasia, Meibomian glanda
Cornea/conjunctiva
Dermoid, ocular
Hyperplasia/hypertrophy, endothelium
Hyperplasia, squamous cell
Filtration angle/trabecular meshwork
Proliferation, trabecular meshwork (TM)Uvea
Hyperplasia, melanocyte
Lens
Hyperplasia, lens epithelium
RPE
Hyperplasia, RPE

x
x
x
x
x
x
x
x
x
x^
x

x
x
x
x
x
x
x
x

a

Terminology with diagnostic criteria and/or comments described in the text.
Terminology addressed in the General Pathology section.
cFinding more frequent as an induced change in inhalation studies.
dCommon artifact.
bComment

Comment

Although not commonly reported as a congenital finding inlaboratory beagles, uveal cysts may be noted in any breed of
dog as a congenital or acquired finding.207 Cysts arise from theposterior iris or ciliary body epithelium and may remain attachedto the point of origin, or they may detach and be observed in theanterior chamber or adhered to the cornea, iris, or lens.208

Most commonly observed in middle-aged or older beagle dogs
as an incidental finding.209 If encountered in a younger population, careful consideration should be made as to whether theobservation is test article-related or spontaneous.

Degeneration, Microcystoid—Eye: Retina

Degeneration, Tapetum Lucidum—Eye:
Retina (Choroid)

Other Term(s)Other Term(s)Not applicable.

Diagnostic Features

One to multiple variably-sized spaces within the inner
nuclear and plexiform layer that disrupt the retinalarchitecture.
Rupture can result in retinal atrophy.

Atrophy, tapetum lucidum.

Diagnostic Features
Changes in tapetal cells may include swelling, vacuolation,
necrosis, or a complete loss of cells.
The alterations in tapetal cells may be associated with
inflammatory changes.

Differential Diagnosis

Differential Diagnosis

Not applicable.

Not applicable.

Woicke et al.

81

Table 16.2. Microscopic Findings in Special Sense Organs: Eye Glands; Dog.
Eye glands
Nonneoplastic
Apoptosisa
Atrophy
Cyst
Cytoplasmic alteration, acinar
Degeneration
Dilatation
Fibrosisa,bHemorrhagea
Hypertrophy
Infiltrate [insert appropriate cell type]aInflammationa
Inflammation, granulomatousa
Karyomegaly
Porphyrin, increased
Regeneration
Single-cell necrosis; necrosisa
Proliferative (nonneoplastic)
Lacrimal gland, glands of the third eyelid
Hyperplasia, acinar cell
Nasolacrimal duct (NLD)
Hyperplasia, epithelium

CommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

x
x

a

Terminology addressed in the General Pathology section.
Terminology with diagnostic criteria and/or comments described in the text.

b

Comment

Eye Glands

Ophthalmoscopic examination of eyes with tapetal degeneration typically shows loss of tapetal reflectivity with the development of focal, mottled, or diffuse zones of pigmentation.210
Spontaneous focal tapetal degeneration in untreated dogs may
rarely be seen,211 but it is more likely to occur as a treatmentrelated change. It has been reported following administration of
a number of different classes of therapeutic agents.210,212,213
Due to the absence of a tapetum lucidum in humans, tapetalchanges in dogs without alterations in other structures of theeye may not be relevant for humans.

The recommended terminology for microscopic lesions
observed in the eye glands of dogs is presented in Table 16.2.

Hyperplasia, Meibomian Gland—Eye: Eyelid

17. Urinary System

Other Term(s)Introduction

Hypertrophy/hyperplasia, Meibomian gland; hypertrophy,
Meibomian gland.

The urinary system pathology has been covered in detail inrodent INHAND document as well as in other publications.
This chapter will provide a brief overview of the anatomicaldifferences in the dog and recommended terminology and diagnostic criteria, along with selected representative images forthe urinary system in the laboratory beagle dogs in preclinical
toxicity studies. Readers are directed to the rodent INHAND
for more in-depth review of histology and physiology of theurinary tract.
Understanding the mechanism, pathogenesis and importance of xenobiotic-induced changes in the urinary system
requires accurate morphologic descriptions of those changes;
therefore, a working understanding of the anatomy and

Diagnostic FeaturesEnlargement of glands with increased number ofsebaceous cells in acini, surrounded by basal(reserve) cells.

Differential Diagnosis
Adenoma, Meibomian gland: Regular architecture is
distorted; basal cells may be prominent and growth pattern exophytic.

Fibrosis: Lacrimal GlandComment
Focal fibrosis of the lacrimal gland, associated with mononuclear or lymphoid infiltrate and/or acinar atrophy, was reported
as an occasional incidental finding in the beagle by Sato et al(Table 16.2).49

82
physiology of the kidney is important. As in the rodent, theurinary system of the dog is comprised of paired kidneys andureters, the urinary bladder, and urethra. The general anatomic
locations of these organs in the retroperitoneum of the lumbar
region of abdomen and pelvic cavity are similar to the rodent
and other mammalian species. There are, however, 2 major
anatomical features unique to the canine, compared to therodent. These are comprised of the complete fusion of medullary pyramids resulting in the formation of a renal crest and the
expansion of the proximal end of the ureters to form the renal
pelvis. The functional consequence of these subtle anatomic
differences is limited and there are no other significant anatomic differences of the canine urinary system (kidneys,ureters, and urinary bladder) compared with the rodent.214,215
Compared with mice, where embryogenesis of the kidney iscomplete at birth, the production and maturation of new
nephrons continues to postnatal day (PND) 10 in rats and PND
21 in dogs.216

KidneyThe nephron, the functional unit of the kidney, consists of therenal corpuscle with the glomerulus, Bowman’s capsule, andthe renal tubule, which encompasses the proximal convoluted
tubule, the loop of Henle, the distal convoluted tubule, and thecollecting tubule or duct which terminates at the renal calyx inthe renal papilla. The nephron is supported by the interstitium.
The counter current exchanger that establishes increasing
osmotic gradient in the medulla interstitium is critical to urine
concentration. The filtration unit of the nephron, the glomerulus, is a complex structure composed of a cluster of fenestrated capillaries closely adhered with visceral epithelia(podocytes) within an expanded sphere (Bowman’s capsule)with adjacent mesangial cells; the 2 cell types contribute to anegatively charged basement membrane which repels positively charged proteins.215
Ultrafiltrate, composed of water, nitrogenous waste products, and small proteins (<60 kDa), exits Bowman’s capsuleand enters the proximal convoluted tubule. “Fetal glomeruli”(ie, immature glomeruli) characterized by small diameter,
decreased matrix, and dense, peripherally placed nuclei withscant Bowman’s space are frequently seen in dogs.49,215 Glomerular filtration rate does not completely reach its maximum
until 8 to 10 weeks of age in dogs, although it is considered
functionally mature by week 6 compared with PND 28 to 32 inmice and PND 42 in rats.217–219 Proximal convoluted tubularepithelium is simple, columnar, and has a prominent microvillus border, which increases surface area for absorption ofwater, proteins, and other substances and secretion of uric acid,urea, creatinine, and other substances (certain xenobiotics).
The inner and outer stripe of the outer medulla is much moredifferentiated in rodents than in humans, dogs, or monkeys.216
Urine concentrating ability develops prenatally in dogs, but
postnatally in mice, rats, and humans.220 The epithelium of theloops of Henle and the distal convoluted tubules are also metabolically active and involved in finely balanced excretion/

Toxicologic Pathology 49(1)absorption of water, depending on current hydration status andelectrolyte concentrations.
The kidneys are primarily exposed to drugs and chemicals
through the blood via the paired renal arteries arising from theabdominal aorta. As such, the kidneys are a site of xenobiotic
biotransformation, which may result in formation of toxic
metabolites, which may be taken up into the cytoplasm of
tubular epithelial and/or excreted in the urine. Renal proximal
tubules are common targets for toxicity due in part to theexpression of transporters that mediate the secretion and reabsorption of xenobiotics.221 Excretion of chemicals and/or metabolites, which may be concentrated in the urine, may result inincreased exposure of the lower urinary system. Finally, thedeposition of biologic molecules (eg, therapeutic antibodies,
immune complexes, or proteins) may occur in the glomerulus.
The juxtaglomerular apparatus is a specialized aspect of theafferent arteriole and the distal convoluted tubule with a keyrole in the renin–angiotensin–aldosterone axis to control systemic blood pressure and is composed of the macula densa,
juxtaglomerular cells, and extraglomerular mesangial cells.215
Erythropoietin is produced by cortical interstitial fibroblasts
associated with the distal convoluted tubules (extraglomerularmesangial cells) in the kidney.
The recommended terminology of microscopic findings in
the kidney of dogs is presented in Table 17.1 (kidney) and 17.2
(renal pelvis).

Immature Glomerulus—Kidney (Figure 17.1
and 17.2)
Other Term(s)Fetal glomerulus.

Pathogenesis/Cell of Origin
Incomplete development of glomerulus (congenital).

Diagnostic Features







Depending on the stage of development, the diagnostic
features may vary:
Immature glomeruli are of smaller size than fully developed glomeruli.
There may be a significantly broader space between thedeveloping capillary tuft and Bowman’s capsule.
Developing glomerular tuft may appear as vesicle,
comma or s-shaped, or capillary loop.
Developing glomerular tuft appears as dense, basophilic
accumulation of cells.
Developing glomerular tuft is lined by dark stained
epithelium cells (podocytes).

Differential Diagnoses
Not applicable.

Woicke et al.

83

Table 17.1. Microscopic Findings in the Urinary System: Kidney; Dog.
KidneyCongenital
Aplasia
Ectopic tissue, adrenal
Hypoplasia
Immature glomerulusa
Renal dysplasia
Nonneoplastic (renal parenchyma)
Accumulation, adipocytes, interstitial
Accumulation, glycogenAccumulation, hyaline droplets
Alpha2u-globulin nephropathy
Amyloid, glomerular
Amyloid, interstitial
Atrophy, glomerulus
Atrophy, tubule
Basophilia, tubule
Basophilic granules
Casts
Chronic progressive nephropathy
Crystals
Cyst
Degeneration, tubule
Dilatation, Bowman’s space
Dilatation, tubule
Edema, interstitialb
Extramedullary hematopoiesis
Fibrosis, interstitialb
Glomerulonephritis
Glomerulopathy, hyaline
Glomerulopathy, mesangioproliferative
Glomerulosclerosis
Hemorrhageb
Hypertrophy, juxtaglomerular cellaHypertrophy, tubule
Inclusion bodies
Infarct
Infiltrate [insert appropriate cell type] interstitiumb
Inflammation, interstitialb
Interstitial nephritisb
Karyomegaly
Lipidosis, glomerulara
Mesangiolysis
Metaplasia, osseous
Metaplasia/hyperplasia, Bowman’s capsule
Microabscess
Mineralizationa,bNecrosisb
Necrosis, papillab
Nephropathy, obstructive
Nephropathy, retrograde
Pigment
Pyelonephritisb
Regeneration, tubule
Single-cell necrosisb,cVacuolation, tubulea,bProliferative (nonneoplastic; renal parenchyma)
Hyperplasia, mesangial
Hyperplasia, juxtaglomerular

CommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
(continued)

84

Toxicologic Pathology 49(1)Table 17.1. (continued)
KidneyCommonUncommonHyperplasia, oncocytic
Hyperplasia, tubule
Nephroblastomatosis
Neoplastic (renal parenchyma)
Mesoblastic nephromaa
Nephroblastomaa

Not observed but potentially relevant

Not applicable

x
x
x
x
x

a

Terminology with diagnostic criteria and/or comments described in the text.
Terminology addressed in the General Pathology section.
cFinding more frequent as an induced change.
b

Table 17.2. Microscopic Findings in the Urinary System: Renal Pelvis; Dog.
Renal pelvis

CommonNonneoplastic
Calculus
Crystalsa
Dilation, pelvis
ErosionInfiltrate [insert appropriate cell type]b
Metaplasia, squamous cell
Mineralization
Pyelonephritis
Ulcer
Proliferative (nonneoplastic)
Hyperplasia, urotheliumab

UncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x

Finding more frequent as an induced change.
Terminology addressed in the General Pathology section.

Comments
In the postnatal period, immature glomeruli are normally present as kidney development is not completed at birth.222 In thekidney of the peripubertal and adult dog, a few immature glomeruli are frequently observed, considered a background finding, and are only reported in toxicologic pathology studies if
their number is significantly increased. They have been
described in conjunction with other congenital renal lesions,such as renal dysplasia and juvenile nephropathy.222–224

Diagnostic FeaturesSpecial Procedures

Hypertrophy, Juxtaglomerular Cell—Kidney
Biological Behavior

Increased size of the afferent arterioles of glomeruli, insevere cases also efferent arterioles.
Invariably associated with hyperplasia of the juxtaglomerular cells.
Increased granularity of the afferent arterioles, that is,renin-containing cells.

Increased granularity may be visualized by Bowie225
and Wilson stains226 and on toluidine blue-stained resinembedded sections.
Increased immunostaining for renin.227
Increased expression of renin messenger RNA.228

Adaptive change

Differential Diagnoses
Sublocation

Not applicable.

CortexComments
Pathogenesis/Cell of Origin
Stimulation of the renin–angiotensin system; increased demand
for renin.

Hypertrophy of the juxtaglomerular cells is in most if not all
instances associated with hyperplasia of juxtaglomerular cells; inthese cases, the combination term hyperplasia/hypertrophy,

Woicke et al.
juxtaglomerular is considered appropriate. The lesions havebeen observed after administration of angiotensin-converting
enzyme inhibitors229 and angiotensin II antagonists.230 Experimentally, the change has also been described following adrenalectomy, sodium depletion, hypertension, and renal ischemia.

85
the lesion reportedly occurs in males and females, it is not age
related, and the severity does not increase with age.

Vacuolation, Tubule—Kidney (Figure 17.5)
Comments

Lipidosis, Glomerular—Kidney (Figure 17.3
and 17.4)
Other Term(s)Glomerular vacuolation, renal glomerular lipidosis.

Pathogenesis/Cell of Origin
Deposition of lipid-containing foam cells in the glomerulus.

Diagnostic Features

Lipid accumulation characterized by deposition of finevacuoles in mesangial cells of glomeruli, giving rise to
foam cells.
Focal or segmental lipid accumulation is often observed
and hyaline droplets deposition frequently accompanies
this change.

Differential Diagnoses
Not applicable.

Comments
Glomerular lipidosis is a segmental change in mesangial cellsof the glomerular tuft with aggregation of lipid-laden foamcells; this lesion did not appear to affect overlying podocytes.231 The foam cells were observed in capillary lumina orwithin the mesangium and distribute focally or segmentallywithin affected glomeruli.49,60,232 The pathogenesis of glomerular foam cell accumulation remains unknown and is likely
multifactorial. Glomerular lipidosis was identified in renal
biopsies from 46 dogs evaluated by the International Veterinary Renal Pathology Service. Glomerular lipidosis was the solediagnosis in 5 of 46 cases (11%), all of which were proteinuric.
All 5 dogs had at least 1 additional clinicopathologic abnormality consistent with renal disease, including hypertension (4),azotemia (3), and/or hypoalbuminemia (2).233

Mineralization—Kidney
Comments
Features of renal corticomedullary tubular mineralization indogs are generally identical to those observed in rodents andare captured in the INHAND publication on the rat and mouseurinary system.234 Mineralization of medullary tubules, similar
to that in the corticomedullary region and similar to that noted
in rats, has also been described in dogs. 49 Shiga et al235
describe vascular mineralization in the renal papilla of dogs;

Vacuoles may be seen in proximal tubule epithelium under circumstances which indicate either physiological or toxicologicalchange. Among species used in toxicity studies, vacuolation ismost commonly observed in beagle dogs as a normal variation,
where both proximal and distal tubules may contain fine lipid
vacuoles,232 and is seen more commonly in females.49,60 The
severity of spontaneous vacuolation of tubules may be increased
by various drugs and chemicals, including vehicles, and may
become degenerative in nature. Vacuolation of the proximal
tubule epithelium due to increased lipid is a rare phenomenon
and is an indication of toxic change.236 Cytoplasmic lipid droplets
can be confirmed by Oil Red-O stain.237 A reversible vacuolationof proximal tubule epithelium (osmotic nephrosis) is seen inlaboratory animals following the administration of hypertonic
sugar solutions, dextran, and polyethylene-conjugated proteins.
Tritium-labeled dextran accumulates in lysosomes, and vacuoles
induced by sugar solutions and dextran at lower severities may
represent reversible lesions. Ionic and nonionic X-ray opaque
contrast media can also induce lysosomal vacuoles in the proximal tubule epithelium, but it usually does not accompany
significant functional or permanent structure damage.232 Administration of cationic amphophilic drugs has been associated withphospholipidosis, an accumulation of exogenous materials withinlysosomes.232 Lysosomes may be considered organelles thatenable detoxification and cell survival by storage and degradationof a foreign material. From this standpoint, lysosome pathology
may indicate cytotoxicity possibly leading to cell death and therefore may be considered adverse, although minimal to mild phospholipidosis is generally considered nonadverse.238

Mesoblastic Nephroma—Kidney(Figures 17.6–17.8)
Other Term(s)Mesenchymal derivative of nephroblastoma.

Pathogenesis/Cell of Origin
Tumor cells of myofibroblastic lineage.

Diagnostic Features






Nonencapsulated and predominantly monomorphic
neoplasm.
Interlacing bundles of homogenous spindle cells withsmall basophilic nuclei and scant eosinophilic cytoplasm.
Widespread myxoid pattern.
Fascicular and wavy patterns are also present.
Matrix full of collagen fibrils.

86

Toxicologic Pathology 49(1)Table 17.3. Microscopic Findings in the Urinary System: Ureter; Dog.
UreterCommonUncommonCongenital
Aplasia, ureter
Nonneoplastic
Calculus
Crystals
Dilatation
Infiltrate [insert appropriate cell type]aMetaplasia, glandular
Metaplasia, squamous cell
Proliferative (nonneoplastic)
Hyperplasia, urotheliuma

Not observed but potentially relevant

Not applicableNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x

Terminology addressed in the General Pathology section.

Table 17.4. Microscopic Findings in the Urinary System: Urethra; Dog.
UrethraCommonUncommonNonneoplastic
Infiltrate [insert appropriate cell type]aMetaplasia, glandular
Metaplasia, squamous cell
Obstruction
Proteinaceous plug
Proliferative (nonneoplastic)
Hyperplasia, urothelium
a

x
x
x
x
x
x

Terminology addressed in the General Pathology section.

Differential diagnoses
Not applicable.

Ureter, Urethra, and Urinary Bladder(Tables 17.3–17.5)

Comment
A rare benign tumor in dogs; the collagen fibrils in the matrix
can be demonstrated by a positive Masson trichrome stain;
immunohistochemically, the tumor cells are positive forvimentin and show myofibroblastic lineage with positive reaction for smooth muscle actin or fibronectin.

Nephroblastoma (Embryonal Nephroma,
Nephroblastomatosis, and Wilms’ Tumor[Human Nomenclature])—Kidney

Degeneration/Regeneration; Smooth Muscle—
Urinary Bladder (Figure 17.9 and 17.10)
Other Term(s)Regeneration, degeneration, detrusor myopathy.

Pathogenesis/Cell of Origin
Degeneration and regeneration of smooth muscle fibers/bundles in the wall of the urinary bladder.

Comment
A rare malignant tumor in dogs, which has been observed in toxicologic pathology studies in beagle dogs. The neoplasm is includedin the INHAND rodent manuscript of the urinary tract.234 For the
diagnostic criteria in dogs of this and other neoplastic findings, referto the fascicle on the urinary tract of the International HistologicalClassification of Tumors of Domestic Animals published by the
Armed Forces Institute of Pathology in conjunction with the American Registry of Pathology and the World Health Organization,239
the chapter on neoplasms of the urinary system240 in the textbook
Tumors of Domestic Animals, and/or current literature.

Diagnostic Features

Myopathic lesions may be complex; consisting of focal
to multifocal areas of smooth muscle fiber degeneration,
atrophy, and/or regeneration.
Regeneration is often characterized by increased cytoplasmic basophilia and nuclear changes (mitoses).
Other features may include individual cell necrosis,
enlarged smooth muscle nuclei and nucleoli, and/or
occasional mitotic figures.

Woicke et al.

87

Table 17.5. Microscopic Findings in the Urinary System: Urinary Bladder; Dog.
Urinary bladder
Congenital
Diverticulum
Remnant, umbilical artery
Nonneoplastic
Angiectasis
Calculus
Crystals
Degeneration/regeneration; smooth musclea
Dilatation
Edemab
ErosionHemorrhageb
Hypertrophy, urothelium
Inclusions, urothelium
Infiltrate [insert appropriate cell type]b
Metaplasia, glandular
Metaplasia, squamous cell
Mineralizationb
Necrosisb
Parasite, nematodeProteinaceous plug
Ulcer
Uropathy, obstructive
Vacuolation, urotheliumb
Proliferative (nonneoplastic)
Hyperplasia, urothelium
Neoplastic
Rhabdomyosarcoma, botryoid typeaabCommonUncommonNot observed but potentially relevant

Not applicable

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

Terminology with diagnostic criteria and/or comments described in the text.
Terminology addressed in the General Pathology Section.

Differential Diagnoses

Pathogenesis/Cell of Origin

Not applicable.

Tumor cells of embryonic myoblasts.

Diagnostic Features
Comments
Degenerative lesions of the urinary bladder muscular tunic,
consisting of perinuclear vacuolation, have been observed invarious dog breeds; additional degenerative features such as
atrophy and foci of necrosis as well as fiber regeneration(cytoplasmic basophilia and mitotic figures) are the hallmarks of the detrusor myopathy, for which the beagle dog
appears to be particularly susceptible.241 In addition, thefinding seems to be more prevalent in dogs with inflammatory changes in the mucosa, arteritis, and generally after
catheterization.241








Nonencapsulated and multilobulated neoplasm arisingin neck region of urinary bladder.
Interlacing bundles of elongated mesenchymal cellswith small basophilic nuclei and scant to moderate filamentous cytoplasm.
Occasional “strap-like” and/or multinucleated cells usually present.
Cell bundles are usually surrounded by a myxomatous
stroma.
Distinct cross-striations may be present in some cellswith phosphotungstic acid hematoxylin stain.

Differential Diagnoses
Rhabdomyosarcoma, Botryoid Type—Urinary Bladder (Figures 17.11–17.13)
Other Term(s)Rhabdomyosarcoma, botryoid embryonal.

Not applicable.

Comment
A rare malignant tumor in juvenile dogs arising in the neckregion of the urinary bladder comprised of embryonic

88
myoblasts.242–245 With embryonal tumor types, including the
botryoid type arising in the urinary bladder, neoplastic cells
may exhibit differentiation ranging from myoblastic to myotubular (ie, multinucleated cells). Neoplastic cells are often

Toxicologic Pathology 49(1)desmin, myoglobin, and vimentin positive.243 Ultrastructurally, myofilaments arranged in parallel arrays or z-band structures are usually present. 243 Distant metastasis has beenreported.246,247

Figure 3.1. Dog, artery, necrosis/inflammation medial or mural, artery, H&E. Courtesy of Eric van Esch. Figure 3.2. Dog, artery, fibrinoid
necrosis, H&E. Courtesy of Eric van Esch. Figure 3.3. Dog, artery, intravascular papillary endothelial hyperplasia, H&E. Figure 3.4. Dog, artery,
intravascular papillary endothelial hyperplasia, H&E. Courtesy of Eric van Esch. Figure 4.1. Dog, oral cavity, papilloma, H&E. Courtesy of Eric vanEsch. Figure 4.2. Dog, oral cavity, papilloma, H&E. Courtesy of Eric van Esch.

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Figure 4.3. Dog, stomach, pylorus, cellularity increased, lymphocytes, H&E. Courtesy of Dr Klaus Weber, AnaPath GmbH. Figure 4.4. Dog,
stomach, edema, H&E. Courtesy of Eric van Esch. Figure 4.5. Dog, stomach, hyperplasia, pylorus (right: normal, left: hyperplasia), H&E. Courtesyof Eric van Esch. Figure 5.1. Dog, pituitary gland, extramedullary hematopoiesis, H&E. Figure 5.2. Dog, thyroid gland, C-cell complex, H&E, lowmagnification. Figure 5.3. Dog, thyroid gland, C-cell complex, H&E, high magnification.

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Toxicologic Pathology 49(1)Figure 5.4. Dog, thyroid gland, adipocyte accumulation, H&E. Figure 5.5. Dog, thyroid gland, adipocyte accumulation, H&E. Courtesy of Eric vanEsch. Figure 5.6. Dog, parathyroid, ectopic cartilage, H&E. Figure 5.7. Dog, parathyroid, ectopic cartilage, H&E. Courtesy of Eric van Esch.
Figure 5.8. Dog, adrenal gland, normal zona glomerulosa, H&E.Figure 5.9. Dog, adrenal gland, hyperplasia/hypertrophy, zona glomerulosa, H&E.

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Figure 5.10. Dog, pancreas, extra-islet endocrine cell proliferation, H&E. Figure 5.11. Dog, pancreas, extra-islet endocrine cell proliferation,
insulin stain. Figure 5.12. Dog, pancreas, extra-islet endocrine cell proliferation, glucagon stain. Figure 5.13. Dog, pancreas, extra-islet
endocrine cell proliferation, cytokeratin IHC. Figure 6.1. Dog, lymph node, erythrocytosis and erythrophagocytosis, H&E, low magnification.
Figure 6.2. Dog, lymph node, erythrocytosis and erythrophagocytosis, H&E, high magnification.

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Toxicologic Pathology 49(1)Figure 6.3. Dog, lymph node, erythrocytosis and erythrophagocytosis, H&E. Courtesy of Eric van Esch. Figure 6.4. Dog, lymph node,vacuolated macrophages, H&E. Figure 6.5. Dog, lymph node, vacuolated macrophages, H&E. Courtesy of Eric van Esch. Figure 6.6. Dog,bone marrow, emperipolesis, H&E. Figure 6.7. Dog, spleen, hemosiderotic plaque, H&E, low magnification. Figure 6.8. Dog, spleen, hemosiderotic plaque, H&E, high magnification.

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Figure 6.9. Dog, lymph node, hyaline material, H&E. Figure 7.1. Dog, liver, congenital vascular anomaly, H&E. Figure 7.2. Dog, liver, ductal
plate anomaly, H&E, low magnification. Figure 7.3. Dog, liver, ductal plate anomaly, H&E, high magnification. Figure 7.4. Dog, liver, bile plugs,
H&E. Figure 7.5. Dog, liver, intranuclear brick inclusions, H&E.

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Toxicologic Pathology 49(1)Figure 7.6. Dog, liver, cytoplasmic rarefaction, H&E. Figure 7.7. Dog, liver, cytoplasmic rarefaction, H&E. Courtesy of Eric van Esch. Figure 7.8.
Dog, liver, vacuolation (fatty change), H&E. Figure 7.9. Dog, liver, vacuolation (phospholipidosis), H&E. Figure 7.10. Dog, liver, vacuolation(phospholipidosis), LAMP2 IHC. Figure 7.11. Dog, liver, hepatocellular adenoma, H&E.

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Figure 7.12. Dog, gallbladder, ectopic tissue, pancreas, H&E. Courtesy of Charlotte Maria Dalsgaard. Figure 7.13. Dog, gallbladder, infarct,
H&E. Figure 7.14. Dog, gallbladder, mucocele, H&E. Figure 7.15. Dog, gallbladder, hyperplasia, cystic mucinous, H&E.

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Toxicologic Pathology 49(1)Figure 8.1. Dog, skin, histiocytoma, H&E. Courtesy of Eric van Esch. Figure 8.2. Dog, skin, subcutis, histiocytoma, H&E, low magnification.
Courtesy of Eric van Esch. Figure 8.3. Dog, skin, subcutis, histiocytoma, H&E, high magnification. Courtesy of Eric van Esch. Figure 10.1. Dog,brain, choroid plexus, extramedullary hematopoiesis, H&E, low magnification. Figure 10.2. Dog, brain, choroid plexus, extramedullary hematopoiesis, H&E, high magnification. Figure 10.3. Dog, brain, ventricle, dilatation.

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Figure 10.4. Dog, brain, meningioangiomatosis, H&E. Figure 10.5. Dog, peripheral nerve, Renaut bodies, H&E. Figure 11.1. Dog, uterus,
segmental cystic hyperplasia, H&E, low magnification. Figure 11.2. Dog, uterus, segmental cystic hyperplasia, H&E, low magnification. Figure 11.3. Dog, uterus, segmental cystic hyperplasia, H&E, higher magnification. Figure 11.4. Dog, uterus, segmental cystic hyperplasia, H&E, highmagnification.

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Toxicologic Pathology 49(1)Figure 12.1. Dog, testis, tubule hypoplasia, H&E, low magnification. Figure 12.2. Dog, testis, tubule hypoplasia, H&E, high magnification.
Figure 12.3. Dog, testis, hypospermatogenesis, H&E, low magnification. Figure 12.4. Dog, testis, hypospermatogenesis, H&E, high magnification. Figure 12.5. Dog, testis, swollen spermatocytes, H&E, low magnification. Figure 12.6. Dog, testis, swollen spermatocytes, H&E, highmagnification.

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Figure 12.7. Dog, prostate, cystic dilatation, H&E. Figure 12.8. Dog, prostate, normal, H&E, macro. Figure 12.9. Dog, prostate, squamous
metaplasia and edema, H&E. Figure 12.10. Dog, prostate, normal, H&E. Figure 12.11. Dog, prostate, squamous metaplasia and edema, H&E.
Figure 12.12. Dog, prostate, squamous metaplasia, H&E, low magnification. Courtesy of Eric van Esch.

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Toxicologic Pathology 49(1)Figure 12.13. Dog, prostate, squamous metaplasia, H&E, high magnification. Courtesy of Eric van Esch. Figure 13.1. Dog, lung, bronchioles,
fibrosis–epithelial hyperplasia, H&E, low magnification. Figure 13.2. Dog, lung, bronchioles, fibrosis–epithelial hyperplasia, H&E, high magnification. Figure 13.3. Dog, lung, alveoli, increased alveolar macrophages, H&E, low magnification. Figure 13.4. Dog, lung, alveoli, increased alveolar
macrophages, H&E, high magnification. Figure 16.1. Dog, conjunctiva, mixed-cell infiltrate, H&E.

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Figure 17.1. Dog, kidney, immature glomeruli, H&E. Figure 17.2. Dog, kidney, immature glomeruli, H&E. Courtesy of Eric van Esch. Figure 17.3. Dog, kidney, glomerular lipidosis, H&E. Figure 17.4. Dog, kidney, glomerular lipidosis, H&E. Courtesy of Eric van Esch. Figure 17.5.
Dog, kidney, tubular vacuolation, H&E. Figure 17.6. Dog, kidney, mesoblastic nephroma, H&E.

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Toxicologic Pathology 49(1)Figure 17.7. Dog, kidney, mesoblastic nephroma, H&E. Figure 17.8. Dog, kidney, mesoblastic nephroma, Alcian blue stain. Figure 17.9. Dog,urinary bladder, smooth muscle degeneration/regeneration, H&E. Figure 17.10. Dog, urinary bladder, smooth muscle degeneration/regeneration, H&E. Courtesy of Eric van Esch.

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Acknowledgments
The authors wish to express their thanks to the memberships of the
BSTP, ESTP, JSTP, and STP for comprehensive reviews, excellent
comments, and helpful edits. And we wish to express our sincere
appreciation to Charlotte Maria Dalsgaard, Eric van Esch, Beth Mahler, Charlotte Keenan, Rupert Kellner, Catherine Schafer, Kenneth
Schafer, and Klaus Weber for their help and contributions in the preparation of this publication.

Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect tothe research, authorship, and/or publication of this article.

Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD
Jochen Woicke
https://orcid.org/0000-0001-7868-6477
Jessica M. Caverly Rae
https://orcid.org/0000-0003-2194-8717
Franck J. Chanut
https://orcid.org/0000-0001-8189-7400
Karyn Colman
https://orcid.org/0000-0002-7054-8544
Maike Huisinga
https://orcid.org/0000-0001-8152-7151
Ramesh C. Kovi
https://orcid.org/0000-0002-3562-2008
Nicholas P. Macri
https://orcid.org/0000-0003-4301-3226
Heike-Antje Marxfeld
https://orcid.org/0000-0001-5390-0633
Ingrid D. Pardo
https://orcid.org/0000-0002-3517-3804
Alok K. Sharma
https://orcid.org/0000-0001-7014-4753
Emily K. Meseck, (GESC Liaison)
https://orcid.org/0000-00022202-6661

References

Figure 17.11. Dog, urinary bladder, rhabdomyosarcoma, botryoid
type, H&E, macro. Figure 17.12. Dog, urinary bladder, rhabdomyosarcoma, botryoid type, H&E, low magnification. Figure 17.13. Dog,urinary bladder, rhabdomyosarcoma, botryoid type, H&E, highmagnification.

Authors’ Note
All procedures used to prepare macroscopic and microscopic imagesof animal specimens for this article were performed in accordancewith regulations and established guidelines for humane treatment ofresearch animals and were reviewed and approved in advance by an
Institutional Animal Care and Use Committee.

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